The main thing you want to do is use a needle size bigger then the yarn typically uses - ie, if the yarn says to knit with size 4 needle, you probalby want to use 6 or even 7. You want the stitches kind of big and loose. I wouldn't bother making a guage swatch in this case, either. Just make your knitted piece plenty taller and longer then 1 or 2 finished size gingerbread men. Once your piece is felted, you can cut out as many men as will fit on it, even making some smaller or bigger then others (so long as you get multiples of 2 each size).
For the felting, use your washer. Put the piece into a zipper mesh or pillowcase bag, use the hot cycle. Add some detergent (what ever kind you use normally) and a pair of jeans or an old beach towel or 2. Every 5 minutes stop the washer and pull out the knitting to see how it looks. Start the wash cycle over if necessary. When it's felted as much as you want it, rinse it out by hand and spread out on a towel to dry.