What a fun project!
This is my favorite with tutorials to help you.
https://www.sailrite.com/
You will probably want home dec or upholstery fabric for the project. I used a fabric that is spill proof for dining room chairs. Easy to wipe off and continue with the day. LOL.
Do you know how to make piping? Put in a zipper? Match fabric patterns? The process is step-by step just like making a quilt. Not hard.
If the homeowner isn't leaving the cushions, ask to measure them before the closing/move. Draw out any unique shapes on heavy craft paper to use for the pattern.