not hard at all. are you familiar with a DMM? if so, the instructions for most (I know all DEI systems) will brief you on how to find which wire is which. Hardest wire in a remote start is the tach wire, but once you've installed one or two that becomes fairly easy. Use google to try and find a wiring schematic for your car.
You'll have to run a wire from the engine bay to your cabin for the hood pinswitch, find your brake wire, your tachometer wire (or if your unit allows for it, you can hook this wire up and use a voltage instead of tach), one or more 12v constant wires, find a place to ground the unit, find an ignition wire, and maybe an accessory wire. the last 3 wires will be large gauge wires (10g or so) located under your drivers side dash and will go up into the steering column/that general area.
you'll strip back the large wires one at a time to see what they are. (in my experience, red is USUALLY 12v constant, orange/yellows/whites/pinks are the ignition and accessory wires). take your dmm, and probe the wire. if it reads ~12v with the keys out of the ignition, in the ignition, and car running(it'll be more like ~14v with the car running), it is a constant 12v source. if it reads ~12v only when the keys are turned forward, and MAINTAINS ~12V THROUGH the cranking cycle when your start your car, it is an ignition wire. if it loses voltage during the cranking cycle and regains it after the car is running, it is an accessory wire. when you find your tach wire (it'll usually be around the coils, or near the guages; but try to find a schematic online; it helps a ton when ur starting out), you'll need to probe it. in a large majority of cars, it will read ~6v, during the cranking cycle and while the engine is running. your brake wire will read ground when the brake is depressed.
Depending what type of car you have, you may need to bypass the factory immobilizer (it keeps the car from running when an unauthorized starting attempt is made, by disabling either the fuel system, ignition system, or both). If you need one of these the shop your working at will provide it.
Any more questions just let me know; I think i covered most points of installing a remote start.
Keyless entry is a little more complex, but not by much. just figuring out the type of doorlock system, how to tap into it, tying into the factory alarm if its there, and some other shit.
go to
http://www.bcae1.com, and read the section about relays and the section about diodes; you'll need to understand them both if you get into more complex installs.