This is how you become a normal sized house, possibly the largest prop in the game, on the map Morgenstund. Becoming the house almost never works for actually hiding, but it can be fun at times.
Finding the Correct Prop Layout
Every map has several Prop Layouts, making it so maps you've been to can still have new and interesting situations, and so hunters can't simply memorize what props are where. On the map Morgenstund, which is the map that is out in the open with several blue houses, there is only one Prop Layout that has the ability for you to become a house. You can tell simply by weither or not the house prop is there, but it's a fair bit away from the witches spawn. You can instead realize if it is the correct prop layout by finding this setup of drinks, planks, and crates directly next to the witches spawn.
Becoming a House
If you are on the correct map and prop layout, you can become the house in the same way you become other props, you just need to get to the House Prop. The House Prop can be found out-of-bounds behind some hedges, in the far corner, left from the hunters spawn, forward and right of the Drinks/Planks/Crates setup. On the right is a picture of where the house prop is on the correct layout, on the left is where it would be if you are on the wrong layout.
You need to get close to the barrier, but not too close or else it will say that you do not have room. It can be kind of fiddly, but if you just spam left click while moving around a little bit you should be put in a space where you will fit. You will know if you are within the reach of becoming it by the white pulsing outline around it. This is a picture of the witch as the house.
Being a House
As mentioned in this guides description, this is really really not likely to work for hiding. Doing this is almost gaurenteed to get you killed. It is just something that you can do for fun if you are bored or can't find any decent hiding place. You can not turn back into a witch, and you can not transform into any other props do to the houses large size. You also can not possess it using the witch skill of the same name. The house has 150 Health, so while you won't die immediately as people notice you, you are so huge that they can't miss.