After looking at that tutorial; here's what I would do...
Fold the house block in half to make a crease down the middle,
then open it up and place right side up.
Then place the triangles right side down and overlap them by 1/8th to 1/4 inch....
so there are two little points up on top. Center these points on the crease, then center the bottom
overlapped pieces on the crease as well. Meaning center the crease in the middle of the overlapped pieces shadow.
That will make it straight, if you need to, pull the point section down to make sure it's centered, then slide it up carefully. Note the distance the points are above the house block and make that the same measurement on all your blocks. Good luck, let me know if that works!