Are they free-standing cushions or attached to the base of the window seat, do you know?? I have not done window seat cushions but have done dining room chair seat cushions. Those were a little tricky because they had a unique corner treatment. I drew a cardboard template of the seat base and took that to my local upholsterer to have them cut me new foam pieces. Measured from bottom back edge to front bottom edge and same process for the sides to see how much fabric/seat I needed. Added in seam allowances (.5" seam allowance on home dec projects).
Used the same cardboard template (plus seam allowances) for my top pieces and then 2" strips for my edges. Made piping. Sewed it all together and stapled to the underside of the seat base.
If they are free-standing/removable cushions, you will need to put a zipper into the back of the edge/side pieces to insert the foam cushion.