You can bind your Tits if they’re large enough to make them tight. You can also clamp nipples for some time before hitting. So clamps on 5 minutes remove rotate 90degrees reattach 5 more minutes. Once times up. Remove and start impact. You can also abrasive the nipples surface prior to impact. Sandpaper or scotch brite.
Same applies to Clit
For the actual impact it self focus on fast consecutive hard hits. The duration and speed usually makes the pain. So think fast drum solo on yourself with a wooden spoon or similar.