Grab stuff, scrap with it.
Do your best to show off the product. Be trustworthy & dependable... don't share files, and keep info from the designer's forum confidential. Be consistent and try to get layouts done before the store goes live. Communicate if you are having trouble using a kit, or are sick and need a week off for some reason. Try to boost the designer's business by sharing your layouts online in galleries, FB, Pinterest, etc.
That's the basics!