You will need:
- IKEA "Slugger" casters $1.99 ( link )
- IKEA "Gorm" shelf $3 ( link )
- at least 8 small screws
- a screwdriver
My IKEA store didn't have the "Slugger" casters in stock so I had to get the more expensive "Rill" casters instead ( link ). Apparently they don't carry them in every store. They're $9.99 so it's coming out at under $13.
Step 1: Attach the casters
Make sure there is enough room between the edge and the wheel so it can turn properly. I measured about an inch (2,5 cm).
The boards are about 0.6 inches (1,5 cm) thick, so don't use screws longer than that or they'll break through to the upper side.