Joe -
You are basically asking us "How do I shoot a bow?".
The ONLY thing a finger sling does is allow you to relax your bow hand without dropping the bow, nothing else changes.
With commercial finger slings the slack is predetermined.
Using a finger sling requires the shooter to have noticeable interphangeal joints on the thumb and index or middle fingers, don't laugh, they are barely noticeable on some people.
A picture is worth a thousand words.
I suggest that once the sling is on, the shooter begin shooting with a FORCED open hand, as in the picture. It will make torque impossible, and get him used to the bow jumping out of his hand, which it will. After he gets used to that, then a more conventional relaxed hand can be resumed.
CAVEAT: finger slings (the plastic loops) can wear out/loosen, usually without warning, before the bow ends up on the floor. For that reason, I usually recommend a wrist sling. The only way it can fail, is if you forget to engage it.
In the above position, the bow should be able to move about 1" forward.
Viper1 out.