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. Repeat for the bottom long wall with coordinates 600,500

After I split the wall, I am guessing it wants me to modify the right hand (NE) section of wall I just modified. Does it want me to modify just the starting co-ordinates, or the ending co-ordinates, or both?
Your help would be appreciated!