Basic AutoCad work

I am designing stairs for my house and I started the drawing in AutoCad but for some reason I cannot find a way to close the shape without altering the dimensions! When you will open the file, you will understand... I believe it is a piece of cake for someone who has decent experience with AutoCad. What I need is someone to close the shape without altering the measurements and explain to me how it is done... There will be a total of around 14 files to finish but if I can do it myself after the first one it is fine, I will still pay the agreed price. Main thing is that I understand how it is done and if I don't then I leave my options open! Thank you for your interest. Patrick