Scratchbuilding

Hi,

Now that my concrete area is complete, I would like to scratchbuild a wagon repair building. I want it to look somewhere along the lines of this: (Small Modern Locomotive Shed K/B - ninth picture down)

http://www.m-s-i-m.co.uk/pikestuff.htm

I want to make it out of plasticard, and I am after a few tips on the subject if anyone has any

Thanks,