Use the Summon Skeleton or Golem ability. Skeletons require corpses to summon. You can summon Skeletal Warriors from the beginning, but begin to summon Skeletal Mages once they are unlocked and you go over your maximum Skeletal Warriors amount. Summoning over your current Skeleton maximum (with Mages if unlocked) creates a short-lived Skeletal Priest who heals and buffs other minions. The Golem also needs to be unlocked, but does not require a corpse. If either summon skill is missing from the hotbar, go to the Skill Assignment screen (default key: S) and assign them from there.