I assume that your flashlight doesn’t have a “battery carrier”. Yes, no?
A picture of a battery carrier shows up in the third picture of the thread linked below. It’s the part with the red arrow on it. Not to make fun, but some people have only used flashlights without battery carriers (in the olden days) :). If you have a battery carrier, that’s where the batteries go. The negative end of the battery is usually contacting the spring in the carrier. Typical battery carriers will need 3-4 batteries.