Some of us that bought the Grip Belt (STP-GB1AM) didnt receive a manual or instructions with it. Why? I dont know.
However, I found a link while reading a DPR Sony SLR Forum thread. It directs you to a PDF file of the instructions of how to install properly the Grip Belt and some things to consider when using it.
This is great news for me since Ive had mine for 1 week already and I havent been able to install it yet.
If you use it with just the camera, you need to keep the tripod mount the grip belt has and attach it to the bottom of the camera. However, if you use a Vertical Grip, you can remove the tripod mount and use just the grip belt.
Also worth mentioning, the metal plate the grip belt has, allows you to install it along with the neck strap. The grip belt works with all Alpha cameras.
Grip Belt Manual
Source and pictures of how to use with the Vertical Grip
P.D. If for whatever reason the link to the manual stops working, I saved a copy of that file. You can mail me if you need it.