It has to to be 6 blocks tall and 7 blocks wide. It needs to be made from wood. It also needs to at least one chair, one table, a torch, and a wood floor.
NOW REMEMBER - You have to unlock all the NPC's. The Guide is the only one that's free. If he never moves in the house then just wait, it takes time. Leave the house for a while. Later, when you come back he will be in the house.
How to unlock NPC's
- Guns Dealer : Find a gun.
- Mechanic - You can find her in Skeletron's Dungeon, just make sure to kill Skeletron before entering.
- Merchant - I think you need at least 50 silver coins,I can't remember how many.
- Demolitionist - You need at least one bomb and the Merchant.
- Nurse - You need at least one life crystal.
- Goblin Tinkerer - After you defeat the Goblin Army look on the map, then head to where he's at. When you first see him he will be called the Bound Goblin. Tap - or click, whatever - on him and he will begin talking to you. Then make the house.
- Dryad - Defeat a boss, King Slime doesn't count.
- Santa Claus - It has to be somewhere around Christmas. That is all I need to explain.
REMEMBER - if you see a message like (NPC's name) needs to settle down, then build a house.