The idea is to hand your stuff over to the guy behind the counter, and he will give you a claim check, often in the form of a bulky plastic card. I wouldn't pass any expensive cameras over. Do be sure to check any coats, sweaters or jackets. Museums are necessarily kept cool and dry, so you will not need any coverings. You want to spend your 5 hours in the museum as unencumbered as possible.
Smaller museums have lockers.
Often there is a one guilder deposit required to use the locker, so carry some pocket change including a one guilder piece. If there is no one attending the locker, it's likely you need to insert the required coin into a slot on the inside or outside of the locker door. Once you have all your gear stashed it's time to check a couple more things before you tour the actual museum. Museums commonly have very passable lunch rooms or cafeterias. I remember having an excellent uijtsmiter at the Frans Halsmuseum in Haarlem.