Unless you have your own vacuum-form table, chances are you’re buying your armour from a vendor.
The pieces will look similar to this when you receive it.

Shoulder bells, gauntlets, and shin guards.

Knee pieces and the belt.

The belt (top shelf), thigh pieces (2nd shelf has the back pieces and the lower shelf has the front), with the full abdominal pieces
(2nd shelf).

All the pieces of the puzzle.

Make sure you have reference documentation available,
before you start cutting.

After you get all your pieces organized, you will need a lot of tools. You can find a really good list here: http://clonetroopers.net/Forum/index.php?showtopic=353

After you get your tools ready, you’ll be doing a lot of cutting. I highly recommend the Dremel tool with the extension. It’ll make everything easier on you.

You should really do this with help, while you wear the unfinished armour pieces get someone to measure where to cut.

I started sizing in my regular clothes and then put on the body suit; you should really measure in your body suit as jeans and other clothes will bunch up on you and not provide the best fit.

Think SAFETY when you cut ABS, chips fly everywhere and
the dust isn’t good for your lungs.

To tighten the fit on the armour, use foam.

When cutting, make sure you are on a good solid surface. You will
have to cut just about every piece you get.

We were referring to the book a lot of the time; besides it takes
two to get dressed the first time.

By cutting the ab piece into two, I can now sit down. This is canon as well.

I also got a pauldron made, which is close to an ARC
Trooper format. It has lamb leather and padding.

Here’s my kamas, made of rawhide leather.

When you’re done, you have it looking something like this.