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.
- Party Girl - Has a fairly rare chance to spawn after having eight NPC's. Also, make sure that the chat doesn't say (NPC's name) needs to settle down or she won't spawn.
- Truffle - Only spawns in Hardmode (activated after the defeating the Wall of Flash). Not only that, he can only spawn in player made surface glowing mushroom biome.
- Pirate - Only spawns in Hardmode. Is unlocked after defeating the Pirate Invasion.
- Wizard - Rare spawn underground after Hardmode is activated.
There are some more but it will take lots of explaining.
REMEMBER - if you see a message like (NPC's name) needs to settle down, then build a house.