You have to combust wood very slowly with as little air as possible.
Traditionally they would make their wood heap and cover it with clay leaving only a very small hole for a chimney and a very small hole to light it with then would stop up or enlarge the holes to keep it at a gently smoulder for a couple of days after getting it well and truly alight.
Then they would stop it up completely a nd leave a few days to go right out then break open and use.