How to assign a Soldier:

Step 1: Sneak.

Step 2: Interact with an Advanced Villager while holding a weapon.

Villager Soldiers can hold many weapons, including Bows, Slingshots, Axes, Battle Axes, Swords, and Spears. They cannot hold legendary weapons.

Make sure they also equip armor by dropping armor in front of them. Soldiers can be rallied up using a Goat Horn.