You can do this the «right» way, by lowering the setting at the foot of the bed (assuming that the crib is already at its highest setting), or you can do this the «easy» way by putting a couple of books or similar sturdy, flat objects under the head of the mattress. (get-your-baby-to-sleep.com)
Push the head of the mattress against the wall, and make sure all bedside tables are far enough away from the bed that your child can't get stuck between the table and the mattress. (newkidscenter.com)
Towels or pillows to raise the head of the mattress, or pillows to raise your toddler or older child's head (babycenter.com)