1. Remove the seat 

2. Decide how far apart you want the studs and how far down from the seat seam you want them 

3. Carefully measure and mark the seat with a white china marker or whatever you have
on hand to indicate the centerpoint of where you will mount the stud 

4. Pull the staples holding the seat material to the plastic base out with a needle nose pliers (the seat material is adhered to the foam in the seat area but not on the sides where the studs will go

5. Take the stud and align it with the center point and push the two prongs of the studs through the material (the prongs are pointed and will push through easily, then fold over the two studs to complete the attachment

6. Staple the material back onto the seat base.

Note: Dectorative  Studs can also be obtained from Tandy Leather or other Craft Suppliers. 

Update: from Senior Swim
I got a few questions from folks about how many studs I put on the seat, how far apart they are, etc.  I ended up putting together a quick pic to show the dimensions better.  As for how many studs, I  put on 23. (One centered at the rear of the seat and 11 on each side).  I stopped where I did because I didn't want the studs in the area where my legs would touch and possibly snag and rip them out.