Plan the job
It looks like your best installation location is the duct work directly above the furnace. The lower section looks like it's sloped a bit, so that isn't a good place to mount. You'll have to use the upper section or cut into the A/C evaporator box. The upper section is probably the better place.
Cut the hole
Once you have a solid plan it's a fairly simple install job. You'll need tools to cut a big rectangular hole. A set of tin snips* will do. You can make a starting hole with a drill, or as I saw an HVAC installer recently do, hold a large flat-blade screwdriver at an angle to the duct and hammer the shaft of the screwdriver until it slices into the metal. This is much faster than drilling and makes a nice big gash where it's easy to get the snips started. It also neatly avoids the problem of chips of metal falling down into the evaporator, heat exchanger, and blower.
Just follow the instructions with the product. Apart from the contortions to get your arms and body into position it should be pretty easy.
The Ecobee 3 can control the humidifier. If you choose to go that route then the control included with the humidifier will go in the "left over parts" bin. You also have to figure out how to wire it on your own; the diagram in the Aprilaire installation doesn't address this situation. I have an Ecobee 3 and an Aprilaire 600. I installed mine so that the humidifier water valve is powered by the 24V transformer in the furnace, ie I did not use the 24V transformer included with the humidifier.
To the best of my recollection I have wires connected to the AUX1 and AUX2 terminals of the Ecobee. The wiring for the humidifier is a series circuit: from furnace R terminal to an Ecobee AUX terminal, from the other AUX terminal to one wire of the humidifier water valve, from the other wire of the water valve to the furnace C terminal. The wiring would be similar if using the Aprilaire 24V transformer -- instead of connecting to furnace R and C, connect to the 24V wires of that transformer.
Actually... as it turns out, I had a new furnace installed a couple weeks ago. I haven't re-installed my humidifier yet but it's on the to-do list. Maybe I'll get it done this week and make an update with respect to the wiring details.
A note about tin snips
In the US, at least, there's a convention of coloring tin snips handles red, green, and yellow. Red will cut straight or curves to the left, yellow cuts straight, and green cuts straight or curves to the right. For years I was a penny-pinching miser and forced myself to use only yellow snips rather than lay out the cash to buy all three. Finally I did, though -- and sheet metal work became SO much easier. They're worth it.