There's a Gun Grip quick start guide that is included in the package, with instructions for assembly.

1. Remove the bottom Bumper:

2. Remove the hand strap and fixed arm:

3. Remove the black Mylar panel covering the contacts on the DR6:

4. insert the bottom of the DR6 into the gun grip and snap it into place.

The gun grip should just start working when the DR6 is on, no app or other configuration is necessary.