The renovate spawn screen allows you to monster spawns to special rooms in your building (the entrance room and the lesser/greater and ordinary spawn type rooms).
The build screen lists all of the full soul gems you have in your inventory that you are high enough level to use.
Choose a soul gem / spawn name then click the button at the bottom of the screen to release this soul into the room and use it as the spawn for the room.
You can use the Despawn choice to remove a spawn from a room. It is not possible to reclaim a soul gem once you have used one.
Spawn rooms
Spawn rooms act similarly to dungeon rooms, once you have placed a spawn room then used a soul gem to place a spawn in the room you can fight that monster spawn repeatedly and gain loot and money in the privacy of your own home.