The following photos illustrate the 3 points of grip, angle of pull, and length of tubing.
Regards the tubing, 7/16 o.d with 1/8 i.d. is common. Adjust the length to a strong yet comfortable pull for you. If you make the sling too strong to pull smoothly, your hand will shake and accuracy will suffer. Note proper grip with the butt of the sling in the heel of the hand(thenar and hypothenar eminences), and proper angle of pull is across your chest with shoulder approx 90 degrees abduction. The weaker sling in the pic is the one I shot the Doormat Hog with in Exuma last July. I used it because I wanted to ensure accuracy since I was in the Freaking Bahamas!, and diving alone. In that instance the spear easily penetrated behind the eye and actually split the right jawbone as it exited(I love remembering that).
Effective spearing with a Sling is extremely rewarding and a skill well worth honing. Some shots I still totally blow with fish at a standstill broadside. And at times I've split the spine going away from 10 ft away. I forget the bad shots and eat well off the good ones!