Not all cribs are standard-sized ... then let it dry before putting the mattress cover and sheets back on. Check the product instructions, though, for guidance on your specific crib mattress.
There are an increasing number of travel cribs on the market these days ... on before you place the included waterproof mattress with a sheet (which is not included) inside.
To test that a sheet is snug, pull on each corner when making baby’s bed to make sure it doesn’t become dislodged. Cribs come with multiple mattress support heights, which basically means that ...