Remove the mattress from the crib.
Assess the height of the crib support for your child's age and activity level and adjust appropriately. The crib support is typically either a panel of metal springs or wooden slats that can be adjusted in height. The highest level is for a newborn. After the baby can roll over, sit and crawl, you should lower the support panel to a medium level. A baby who can stand is safest with the support set at the lowest level.
Note that some cribs have a snap-and-lock system for the mattress support while others need to be secured in place with screws. Use the appropriate tool to loosen and adjust the support, and to tighten it after you make the adjustment. Depending on the manufacturer and model, you may need a screwdriver or Allen wrench to adjust the mattress support.
Spread the crib skirt across the mattress support with the right side of the fabric facing up. Push the drop sides of the crib skirt between the edge of the mattress support and the sides or frame of the crib. Pull the crib out from the wall and arrange the crib skirt around the mattress support on all sides so the hem hangs evenly from the floor.
Push the crib back into place and replace the crib mattress.