Start by making a bunch of things out of paper, don't worry about getting scaling down Just yet. The reason for that is so you know what to do and how to do it later on.
Once you've figured out how to assemble the parts, try messing around with the scaling. Make them BIG! make them small. THEN when you've got a basic handle on scaling, try to get the scale correct.
I've said it before and I'll say it again, 405th.com is the right place to come, but it's a crock pot, not a microwave. It takes time. As a result of that time you spent, you get a quality, custom made costume or prop. Not a cheap imitation. Totally worth it. And this forum is amazing for inspiration, resources, friends, and building tips.
P.S., if you live in the area (Idaho, Utah, Arizona, Nevada, Wyoming, or Colorado) join the mountain regiment. We're pretty much the best since ever and are totally bomb.
... if you don't, then second best is ok, too!
(jk, the other regiments are awesome, I never would seriously put them down.)