99 Nights in the Forest campfire: fuel it, upgrade it and keep a reserve

For the first few nights, collect wood close to camp and keep a little fuel beside the fire. The campfire protects the area and opens more of the map as it improves, so don't spend the whole day exploring and return to an empty fuel bar.

From tree to fire

1. Use your axe on a nearby tree and bring the logs back to the campfire. Keep track of the route so you can return before nightfall.
2. Pick up a log and place it onto the wood pile under the campfire flames. Aim for the fire rather than the separate grinder or crafting station. Check the bar to see whether the fuel was accepted.
3. Feed it more fuel as needed. Logs are a practical early option; coal and fuel canisters found while exploring can also be used. Keep some at camp for later instead of carrying every piece away.
4. The campfire upgrades when its progress bar reaches full. Higher levels require more fuel, so bring back stronger fuel when you find it rather than relying on logs alone indefinitely.

At campfire level 4 and above, ordinary logs contribute very little. If available, save logs for crafting and use coal or fuel canisters to advance the fire; keep enough fuel for emergencies.

The bar is low and it's almost dark

Return to camp first. Use the reserve on the fire and collect nearby wood if you can do so safely. If you're playing with others, ask one player to stay close and watch the bar while the others return. If the fire goes out, restore it as soon as you can; leaving camp exposed at night is dangerous.

Fuel needs and available items can change with player count and updates. Watch the in-game bar rather than treating a fixed number of logs as a universal requirement.

Sources: community campfire reference, fuel placement guide.