Remove the sheets from the crib mattress. Remove the mattress from the crib and set it aside. You do not need to purchase a new mattress.
Locate the two bolts holding the drop rail or footboard in place. Remove the bolts with an Allen wrench.
Lower the footboard or drop rail to the floor and pull it forward, out of the crib frame. Lower the mattress support to the lowest setting.
Raise the toddler rail into the crib frame so that the bolt holes in the rail line up with the bolt holes in the frame. Insert the bolts into the holes in the frame and tighten with an Allen wrench. If your toddler rail comes with a wing nut or bolt cover, install those as well.
Replace the mattress, sheets and other bedding.