I made an account because I saw this question, First I hope I don't forget and never come back here again, and second, depends on many conditions. I have experience working for 5 different company dealing with protective coatings, specifically, or managed that department. Most only lasted a year or so, except one 6 years, because it paid the best and was the most professional, and knowledgeable employees. The corporation is structured to have the best products, on the way thing are developed and their labs and chemists, and testing facilities. Other places lie, or will give you false advise knowingly or unknowingly, in my personal experience. I also own my own businesses for power washing and painting, especially floors. I have worked with another painter as a partner for 2 years and a while with an epoxy Apoxy company after leaving the sales end of things.
I don't usually recommend a floor grinder depending upon the condition of the floor. If it's a new house, with smooth concrete or it is pretty good, just some divots here and their it's alright and doesn't need grinding. Grinding won't hurt but with the right products, self leveling agents are in the product. Grinding Is not needed, unless it's like a driveway/sidewalk, or just cracked and brittle (be careful if you do power wash). Doing manufacturers floors many times, we never needed to grind the entire floor, actually in about a year only needed it once. Products now from the right place will level themselves, also tools are made for that. I don't think a grinder is needed, or at least in most especially homeowner situations. If you do that would be before power washing.
It depends on the condition of the concrete floor as well as product you are using: what the chemical makeup is (active ingredients in protective coatings are added to do one job). So step by step this is the correct and most safe way to have the job done the right way, one time and not waste money having to re apply/buy more product. Also their are Concrete floor epoxy kits and some that are already combined. So I'll walk you through the steps. As far as weather goes, make sure you know the temperature the product can be applied and how long it takes to cure/dry 100% so it stays in those ranges. Of course rain is also a factor, but you're in a garage. However opening the garage door will help the process of the epoxy, any air flow is a great help. Follow instructions for that specific product, there should be data sheets with all the info you can imagine, it is actually a law to have them. Some place actively will hand one out, if your wanting a epoxy usually not for homeowners, more industrial grade. I will mention this a few more times probably stay away from H&C Kits (blue Box).
Analyze the concrete, look for darker areas on the bare surface. Is their any coats on top, an enamel either in good shape or almost worn away. Also a good idea is to figure out if there may have been any protective coats, or additives that would reduce mold, mildew even insects. Just knowing what the ground has been treated with in the past is always beneficial. You never know how two things will react. An old treatment to the concrete may still be present and cause a reactions, bubbles, peeling etc. Especially with the "wet ground" and "Glaze" look people usually get for small sidewalks, around pools and patios. Being a garage, and enamel is most likely (unless it is some industrial garage, then Apoxy or Epoxy)
Cleaning When was it built/laid (the floor), is the concrete in good condition, hard not brittle and chipping. This leads to the first possible and recommended things to do. Powerwash the floor to get it clean as possible. This will do the best cleaning, even add different agents made for concrete, some are sold at Sherwin Williams in a white 2.5 Gallon Jug. These help strip away any fungi (Darker areas etc.). Products like this are recommended for the best outcome and adhesive properties of the product.
What product suits your needs? You don't want to spend $300 or much more per gallon. However that is easily possible, also not getting much square footage out of those more expensive products. Pick a product, any good epoxy that will sustain cars pulling in the driveway that will last, was minimum $100 in my experience. Get contractor grade things not things for (home owners, many common stores if not all I would suggest this, unless it is a specificity Protective Industrial or Commercial Coating supplier with nothing but industrial grade stuff, that will run much more money usually).
Go to the store and purchase it, I recommend Sherwin Williams. Don't let them sell you the H&C set, have something from the back, many contractor grade products are in the warehouse, actually the entire back is filled with them, as that's their main revenue (Most Locations, maybe different for newer stores). If they don't something their they can order it. Stay away from Lowes and Home Depot. Sherwin Williams will have the most actual educated employees, find a Full Time Employee, Assistant Manager or Manager. Sometimes the full time employee will know the most (as they may have worked their the longest just no promotion because no 4 year degree). They have to learn the products and take hundreds of tests about 50-60 the 3 months working their. Also access to data sheets, and maybe a contractor in their to give their advice as well, techniques, the employee may have other techniques, so you can do it the way you would comfortable with. Usually the employees will tell you the correct way. and the way that the product was made to do and perform, doing it this way also ensures a warranty.
Once cleaned, prep the rest, tape if you want, start in the back. Depending on the product certain brushes will work much better, however I would roll most if not all, if possible. At Sherwin Williams, Maybe PPG or Ben Moore they will know the correct roller to use (also one sherwins data sheets, if employee does not know off the top of their head 1000s of products). Just roll it out, get a pole depending on budget the Purdy extensions are nice, however expensive. I would just buy a 4 or 6 foot wooden standard pool or a cheap extension pole (it is the floor so gravity isn't against you). Just roll, certain place will have (if not they will get more in) business card sized pieces of some alloy like steel almost or Nickle, it will have points coming out of some sides, and tiny ruler like measurements, these are used to make sure the coat is the correct depth (usually one coat for epoxy, unless it's not at the correct level or in the right range, then add more while it is still wet, before any dries so it bonds as one solid layer of protection.
If you have a garage and it's for normal household use. No huge heavy forklifts or construction vehicles driving on it, of it no cars at all, go with an enamel. A specific enamel great for smooth concrete, can be driven on, however will wear with time is Poarch and floor enamel. It's water based, so easy clean up and quick drying time. If you want an additive made of almond shaving, however look like sand can be put in each gallon and the more you add the more will stick together and create a course non-slip surface. Also the more coats the more course it will be. Stick with a light color as the darker the color a different base is used (this is true with all Paints, enamels, Apoxy and epoxys that have color added at the location of purchase), once it is in a medium tone and needs a "deep base" less solids than the Extra White, Ultra White or Just White depending on place of purchase and product. If you want the durability and look of a commercial floor covering and want it strong enough to drive a tank over many times that is fine, I would do it but it can get very pricey and some only 150 Sq Ft. a gallon. There are discounts for being a business owner/contractor with an account, or knowing a painter with one always helps asking the employee if they know any contractors who don't mind letting people use their account may work as well. Creating or having an account of your own at certain retailers, can help a lot, but generally for painters, builders etc, real estate agents have started them even doctors offices who need to paint a whole building and doing it them selves. If you are thinking of getting a 2 part that comes in a blue box with the flakes, in red and black, or green, black and beige (can't remember the name), part a and b, Gallon and Quart (how most are), however this is in a user/homeowner friendly box, not in the back where the "contractor grade supplies usually would be located, at most retailers. I would stay away from Home Depot and Lowes and not listen to a word the employee says no matter how confident they sound. For example tell you primer isn't needed. Even when talking about bare dry wall. or Paint and Primer in one, this is all a lie. Primer is always needed unless it has been primed and painted before, just scuff up the walls so they are less shiny than the sheen of paint applying. Usually when company or product says "paint and primer in one" it is almost false advertisement to most consumers. This means it will have a good hide, or cover up what under it. How much it covers is different, that's related to Square Feet. Witch average paint will give you 400 to 450 a gallon. The Porch and Floor enamel I suggest if it's not being beat up like an industrial warehouse is ~350 and shelf cost $55 a gallon, at least a year ago, but automatically gonna get 10% off and very good chance 30% or 40% off, depending on when you want to buy it. A good epoxy is going to cost at least about $100 that will stand up to cars, also with epoxy, make sure it is the correct depth, it should be on the can, or ask provider for data sheet (they need them anyway for law)
Yeah, could go into techniques but that would be hard to explain, instead of showing.