(Add the iconic Red Phone Booth to your Lego Town/City or display. All NEW parts - Custom Model - Comes in clear zip-lock bag with building instructions. We always try and answer queries quickly and concisely :-).
https://flic.kr/p/8VLv1w | detail on angled building on slope | It's all about collecting a bunch of slopes and combining them in the right quantities. (I instictively understand Pythagoras better than I can explain.)