use an over hand OK grip, press mostly on the sides behind the glans. Use something to ensure the grip is good (cloth wrap, dishwashing glove, talc).
Look at V-stretches (in the FAQ), JAI stretches and DLD blasters as a way of increasing power and effectiveness.
Upward angles will have more effect on the tunica. Downard angles through to BTC will impact the ligs more. The ligs should be easier to stretch so BTC is seen as highly effective but it may impact on erection angle.
For a simple inverted v-stretch: while sitting pad the front of the chair and stretch over it with the mid point of the shaft receiving upward force from the chair edge as you pull down over the edge.