They're usually mounted on the port side, but it really dosen't matter, it can be on either side.
There are 2 styles - outboard and i/o, (see links below). The outboard model has one upper brace and the i/o model as 2. If you have a notched transom, you might have to use an outboard model and mount it with the upper brace toward the outside of the hull, (no brace toward the motor) depends on clearance. If it's a full transom, with bracket, you can use the i/o, (2 brace) model, as shown above. I prefer the i/o style, it has more strength and is easier to board. Also, some platforms come with no ladder, (you add one) and some come with a single or 2 step ladder. I prefer at least 2 steps, again... easier boarding.
The platform gets thru-bolted. The transom is aproximately 3 inch's think, so you'll want bolts at least 4 inch's long, (if they're not already included). Use washers inside and out, and lock nuts. Make sure you thoroughly seal every thing with either 3m's 4200, 5200 or LifeSeal.
The tricky part is getting the height right. You want the platform itself just a few of inch's above the water, when at rest. The higher the platform, the more difficult boarding will be. If you have Bennet trim tabs, (depending on the platform) you can only go so low, without hitting the top of the actuator. This is actually a pretty good refernce point, (see pic). Anyway, once you have your desired location, hold the assembled platform against the transom and mark it. Measure down from the gunel, (use a square, or lay a 2 x 4 on top) to your marks. Now look inside the transom area and compare the measurements to make sure your not obstructed. If you have a second battery or an oil injection tank, they may have to be temporarily removed. Also, there's a ledge on the inside of the transom that may inhibit the location of the upper platform bolts, so watch out for that.
Most after market boating stores carry them, (West Marine, Overtons, etc).
BoatFix -
http://www.boatfix.com/bypage2index.asp?page=427 Click on "page 427"... (pdf). Actual price is on 1st page.
Overton's, (i/o model) -
http://www.overtons.com/modperl/product ... 009&r=view
Overton's, (outboard model) -
http://www.overtons.com/modperl/product ... 009&r=view
West Marine -
http://www.westmarine.com/webapp/wcs/st ... assNum=162