Did this last year on my own boat.
Used a Garelick(spelling?) mount.
Washers or plate? The best method is to use a backing plate especially as you are using a four cycle kicker (vibration). The problem then becomes, how do you assure that the holes you drill in the transom are exactly 90 degrees to the transom surface coz, if they aint, the backing plate that you patterned off the kicker mount wont fit. What you could do is make a paper or cardboard pattern off the bolts on the inside of the transom after the holes have been drilled then the backing plate will fit better. Try to make the holes in the backing plate only big enough to allow the bolts to pass through. Make the backing plate from stainless at least 3/16 inch thick so it wont crush when you suck the bolts up. . . and it should be bonded electrically to your engine block. . . and the plate and the stainless mounting bolts should be bedded in with 3m 5200. . . I would use stainless nylock type of nuts as well. What you're trying to do here is prevent the nuts and bolts from coming loose, spread the mounting load evenly without crushing the transom and causing leaks.
How high to mount it? The instructions that came with the mount should cover that, no? If not, you could look at Garelick's website and download their instructions. . .. otherwise, you need to keep the prop of the kicker deep enough in the water so as not to suck air or cavitate. . . keep it clear of the trim tabs if you have any. . . and low enough to get into clear water underneath the hull of your boat. . . and when it's in the raised position, out of the water when you're using the main engine.
You also have to consider how you're going to get fuel to the kicker too . . . direct tap into the main engines fuel line?
How about also connecting the kickers alternator into the boats electrical system to keep the batteries up while you're using your fish finder, stereo, etc.
Hope this helps!