That's what I do, very simple !It is easily explained.
1. Wrap your slingshot handle in thick rubber or cloth.
2. Clamp it in the vise, at a position similar to your grip.
3. Attach rope instead of rubber bands
4. Grab the rope with both hands and pull as hard as you can.
A slingshot frame that survives that test will be safe to shoot.
Jörg