Build Necromancy school in the capital to unlock the "Summon Skeleton" spell. Hire a Hero and give him the spell in magic slots. Level up the hero to unlock Necromancy skill. Mage can get it after 1st level up, other heros can select Necromancy specialization at Level 10. Summon Skeleton in a battle and make sure it is still standing (has some HP) after win. Easy targets to summon skeletons are battles with bandits, orks and similar. If hero has empty slot(s) in the army, there will be an option to add skeleton(s) permanently (requires Necromancy skill). Sometimes (but rare) heros summon skeletons in auto-battle and can still add them to empty slot if win. Remove skeleton(s) from the army to get some empty slots and repeat steps 3-5 (grind).