Most decor patterns are in standard sizes for things that come in standard sizes. Window seat cushions tend to fall in a custom make catagory & you won't find your precise size. A pattern could give you insight & instructions but you will have to change pattern pieces to accomodate your correct size. If it is a standard rectangle, you may do better getting a book with instructions & forget patterns all together. The top & bottom are simple to cut but you may have trouble with the side pieces with the zippers. You can buy foam from 2-6 inches or so thick. Your top & bottom will remain the same regardless of the thickness you select. The fronts & sides will be a strip of fabric the width of your foam plus a seam allowance on each side. The back & back portions of the side panels will be two pieces that will have a zipper in the center. Once the zipper is in, it will be the same width as your single strip. You can look at existing cushions & see what I am referring to. It is not hard but involves some math & concentration. Welting is made from bias strips where you insert cord, fold the fabric around it & sew it with a zipper foot as close to the cord as possible. Attach it to the top & bottom pieces (with your zipper foot) before you attach the side bands.

Joansews4u's suggestion to get a book is a good one. Singer publishes some nice ones with good illustrations and picutres. You can probably borrow one from the library, if you don't want to buy it.

Here's a link to instructions for just the cushion you want to make. The instructions are complete, but don't include a lot of details or pictures, but this gives you and idea what it involved.

CMC

Here is a link that might be useful: Box cushion

You can also find directions here:

http://www.upholster.com/howto/cushion.html

There is also a forum on that site where pros can answer your questions. I've done 2, 1 more to go.