The best way to e-stim a cock is using a small plastic/resin chastity cage with bars, and running conductive rubber loops through the bars in various places.
The cage, as long as it is small enough, will hold the electrodes firmly in place at all times even with (partial) hardening and softening of the cock - this is a problem with other methods of cock e-stim. Holding the electrodes firmly in place prevents burns.
Best setup I've found is one electrode on each side of the glans and one electrode on the middle of the glans - run current between the two electrodes on the side of the glans for panic-inducingly intense edging. The electrode on the middle of the glans runs current to either a testicle electrode or an anal electrode. Glans only stim = edging. Glans stim + glans to nuts/anal stim at the same time = cummies.
Nuts to anal stim produces a uniquely horrifying sensation that masochists will love.