My mom has a bunk bed at their 'beach house'. It is really difficult to make. Someone has to crawl up there and try and 'tuck' it in around the mattress which is really snug up against the frame without bashing your head on the ceiling! Ooof. It definitely doesn't go 'under' the mattress. I was happy when the 'kid' was old enough to help out!! I like the idea of a 6-7 drop on the sides and perhaps even that on the end. Maybe you should ask if the mattress is a standard size? (My mom has a purchased blanket from Tuesday morning.)