Thank you Hansmex. I'm following this guide but one of the parts is confusing me, and keeping me from moving forward. Could you help?
In step 2, it says: 2 - Select the top long wall, then right-click → Split walls to split the wall, then rightclick → Modify walls to change the coordinates to 600,0 ...

I am trying to recreate an interior room so I can design a storage system for it. This is on the second floor of my older home. The walls go up vertically partway, then angle in towards the centre of the room.

Is it possible to recreate this? I have seen the sloping wall feature, but playing with ...