Biggest thing is keeping your strings from twisting.
Make your string with the loop shorter than the other. Loop it around your middle finger. Hold your other strings between your thumb and index. This will keep your strings from twisting.
Actual technique varies. I use a 24" sling and spin it about 60rpm until launch. The last full rotation I pick it up and it clocks between 120 and 140rpm. Launching at this speed I have to open my hand at about 120* from 12-o'clock. Slower rotations will leave you opening later, fast you will need to open sooner.
Just like archery, it takes practice, practice, and more practice.