1
clean out under your bed
2
get a mattress that will fit underneath
3
get some extra sheets
4
put the sheets on the mattress
put the mattress under the bed
6
OPTIONAL - put a bed-skirt on your bed so you can't see the mattress
7
keep some extra pillows and blankets around for when friends come over
8
when friends come over pull out the bed underneath that way they'll enjoy sleeping over because they don't have to sleep on the floor