There has been a previous Ible describing how to make a trailer-style leaf catcher for a riding mower. http://www.instructables.com/id/Leaf-catcher-2000/
This Ible describes a grass/leaf catcher that sits on a platform attached to the rear of the mower. This is the same idea, different approach.
Step 1: Can Your Mower Use A Grass catcher?
Not all mower decks have the necessary outlet where you can attach the grass catcher. Mine did have an opening in the side of the deck where I could attach my fitting.
If your deck doesn't have an opening then you are going to have to cut an opening to make this work. That is a much bigger project and will leaved your mower permanently modified.
Step 2: Remove Any Covers
My deck had a spring-loaded plastic cover over a pivoting metal pate. I removed the pivot pin, the plastic cover and the spring. I kept the metal plate and put a 4" long 1/4" hex bolt through the mounting tabs to hold the plate in position.