I added valves to my kayaking drysuit, I bought Apeks (very happy withh them) valves, from the UK online, and bought the backing patches.... the assembly goes like:
Roughen the patches well with sandpaper and clean thoroughly (NO GREASE)
put on suit and lie in your normal diving position on teh floor
have an assistant mark the upper part of your left arm with an x for the center of the valve... stay away from seams in teh suit.
glue on the patch to the outside of teh suit using a compatible glue for both components
allow to set up good and propper (24h)
cut out the material in the center
fit the valve as per the enclosed instructions (supplied with valve - READ THEM)
go diving and see if you did a good job on the glueing or not!
I fitted my valve exactly on the side of my arm... I would move it back about 30 degrees if I did it again so that i didn't have to conciously roll to allow it to dump air.
have fun
R