Stand holding a sturdy bed sheet with both hands and lean slightly forward at the hips with a straight back. With a slight bend in your elbows, pull the sheet outward and slightly backward in a controlled motion, keeping tension on the sheet. Focus on squeezing your shoulder blades together and contracting the rear delts at the top of the movement, then slowly return to the starting position.
Log your rear delt fly with bed sheet sets in 1RM.fit and watch your estimated 1RM update automatically as you get stronger.