I recently put a couple of four inch sections on both sides of a fiberglass '14 stock. Got the rails from the ergo grip people, thirty bucks each (ouch). Saw them a bit cheaper elsewhere though. Pretty simple, just get it all laid out and mocked up for sure as far as location and alignment before you start drilling holes. To get the rails aligned with the line of sight and bore, had to shim the front of each out a bit or else they will taper along with the foreend of the stock. Just use some calipers til you have an equal or very close to equal width front and rear on the rails. I used a couple of washers under the front of each rail and just filed them thinner until the width was right.
Once I got everyting lined up as needed, I loctited the hardware in place then used some of those strips of putty that are used to hang shit on walls and made a dam on three sides of the rail. Then used some five minute epoxy to fill in the gap that results from the rail not laying flat against the foreend. Took a couple extra applications of the epoxy to fill in every little void , then sanded the epoxy to give a molded look to the stock and rail. Course, if you sand enough to give the best blend of rail and stock, you will sand into both the stock and rail enough you'll want to repaint them.