Here are the basics for adding a seat to the BEKVÄM step stool:
1. Cut a board and piece of foam to the size of the top of the stool
2. Cut fabric large enough to fold around the cushion and board; staple along the bottom.
3. Attach a wooden plug that fits through the handle hole of the stool to avoid the seat from slipping, but allows it to be removed when needing a step up.
You can read more details here.