The McCallum drone valves are designed for the modern range of synthetic bags from Bannatyne, Canmore and Ross. They give you better control during intros, and have proved successful in getting that clean cut off. This valve is not suitable for traditional skin bags.
Step 1: Fine Adjustment: The valve is pre-set at the factory. If you feel you need to make adjustments, gently pull on the elastic loop and move the small rubber sop (shown at arrow below) up or down to lessen or increase the tension. Some trial and error will be necessary, but please note that a very minute adjustment is necessary.
Step 2: Hemp to Size: The valve as been designed to fit most drone stocks, but some hemping might be required. Black waxed hemp is recommended
Black waxed hemp can be found here: https://www.jhiggins.net/hemp-waxed-black/
Step 3: Fitting: Once fitted with hemp, your valve is ready to be inserted into the stocks (or the tubes for a Ross bag). If either the Bannatyne or Canmore zip bags is being used, then the valves can be fitted without removing the stocks.