First off you need to chop the grip part. I wanted mine very small, so I started at the highest groove on the grip. You can adjust this for how large you want your grip by either cutting further up the handle, or by cutting in a lower groove, I find the highest grove gave me the best results.
Here is what I cut out in red
After your done cutting out the body, save it, you'll use it later. Now for the piece in blue. What I did was I turned that piece upside down, and found it fit perfectly over the now nub at the very top of the vfg. I started with gorilla glue, and glued it into place, and then went over and plastic welded the grip piece, onto the top of the VFG. I used the body of the VFG that I mentioned before for the extra plastic needed in the plastic welding process. If you are careful, you can make it really uniform and neat, making it look as though this was how it was made, mine came out close to perfect.
Finally the last and hardest piece. The core. I originally chopped the core directly above the threads, and also about 1/4th inch from the top/nub. When I tried to get the core into the grip I found it didn't fit, and had to grind off about 1/2" of the threads on the core. I got it all to set well, plastic welded the top/nub to the bottom of the core, and got what you see today!
I can try and get more pictures for you, but right now I'm at work, let me know if your interested, and give me 8ish hours