Building a campfire in Valheim is very easy. You need to collect 5 x Stone and 2 x Wood and equip yourself with a hammer.
To craft a hammer you need to collect 3 x Wood and 2 x Stone. Here is the step-by-step guide to craft a hammer. Click
Once you craft/equip with a hammer, right-click (Mouse-2) to open the build menu. Go to the “Misc” tab and select a campfire using the left mouse button. Place the campfire on some flat surfaces like Ground, Flat Stone, etc. by clicking the left mouse button.
Note: Unequipped the hammer if you don’t want to place multiple campfires.