Go to Sears and get a 4x36 sander, it will come with a 220 grit belt. Get some scrap to practice on. You will need to get a set of Mike's to check the thickness as you work the piece down to the proper size.
If it is hard to hold utilize some masking tape and tape it to the surface to hold on to. Do not apply pressure when working down the material. Just hold it steady and let the sander do the work. Mike it at first, sand a little and check. When you get it down to the proper thickness then stop.
Also, check your material on a granite stone for flatness. You can always use the sander on other projects out in the shop. It will be a good investment and not to expenses. And use some kind of mask when you are sanding Micarta. If you want to see what one looks like I have one on my site in my shop.
Have a great day and remember that it is not the only way, its just one way to do it. Hope this was of help to you. :thumbup: