I found some old fabric in my basement that I don't care about and made my own pattern out of it. I kept refitting it -- pinning and what not -- until it fit me the way I wanted it to. Then I made them as neat as I could so to use them as patterns for the actual vest. I bought the fabric that I wanted, which was black vinyl-like material, and cut out the vest from it. I sewed it up, putting a zipper on one side so I could get it on and off easily. After that, I decided I would attach my plates with velcro. On the plates I use Industrial Strength sticky back velcro but used regular sew-on velcro on the vest (sticky back velcro doesn't agree with sewing machines). The only real difference I could see in the industrial velcro and the regular stuff was the side with the hooks, as apposed to the more "furry" side. The industrial velcro's hooks were stronger and more organized than that of the regular stuff. So I used that in sticky on the plates and the regular "fluffy" on the vest. To position the plates, I put on the vest, marked the plates with chalk and then kind of rubbed the chalk onto the vest (chalk wont hurt the fabric at all and it isn't permanent so it's easy to reposition). Then I applied the velcro where necessary.
I hope that gives you some ideas!