Since we're in december, do not pour concrete when it freezes. If the weather is too cold it will take longer to set, and if it freezes, it will crumble.
Dig hole. Try not to loosen the dirt too much below the bottom of the hole, otherwise it will compact later when you park the vehicle on it and the slab will sink in.
Pour a layer about 2-3" thick large gravel on the bottom. This is mostly for drainage, to keep your concrete dry so it doesn't freeze and crack in winter. It'll also give more even support.
If you want it to look nice, make a rectangular form with some planks. You can oil them with rapeseed oil.
Put in some mesh rebar. You'll need a bolt cutter, or a hacksaw and lots of sweat. It should be in the middle of the concrete, because if it is too close to the edges, it will rust, expand, and crack your concrete from the inside. So you can mix a bit of concrete to make a few heaps and rest the rebar on them.
Mix concrete. If you use a cement mixer, you can throw some reinforcing fiber into the concrete for extra strength. If you mix it by hand, only use fiber if you have big arms and enjoy a good workout.
Pour concrete, work it with trowel, etc. Make sure there are no holes, "vibrate" it with the trowel to help it set, hit the planks with a hammer, etc. Basically vibrations help it set and fill voids.
Next, do the surface so it looks nice. There must be a slope so water drains, so you can make it a bit higher in the middle. If you expect it to sink into the ground a bit, you can make it higher by about 1/2", but then if it doesn't sink, watch your toes when you walk around.
Next wait long enough for it to set, and remove planks.