I thought I had banged on about nettles until everyone was bored rigid. However I received this question in a comment.
P.S. Are you able / willing to share a photo of the type of nettles to look out for please?
So I hope this will be a definitive post.
The best nettles are bright, bottle green in colour and have yet to flower. The leaves are close together at the top, which is a fantastic side effect of them being at the right stage; when it comes to whipping or stroking with them. This is the Springtime nettle. These sting the most by far and the stings can last for 4 hours.
As we move to summer the nettles flower, (the strings of bobbles). The colour becomes more grey and the leaves are spread out along the stem. These sting the least and the stings last the shortest amount of time. The spread out leaves are unhelpful too.
If flowering stems are cut down, new green shoots grow from the remaining stem stub, and later in summer new shoots come from the ground. These two types tend to have weak slender stems, but are almost as good as the spring, pre-flowering stage. Always opt for weak, floppy stemmed new shoots over sturdy, grey, already flowered old shoots!
Remove some leaves from the stem for a handle to grip. Whipping with nettles works best, followed by constant stroking at the same location, before moving on. Reagrding the penis, the head is more painful that the shaft and the closer to the urethra opening the more intense the pain – this can be incredibly intense very close to the opening, compared to the shaft stings.
I find the most psychologically impactful is just to sting the penis (and perhaps the scrotum), but not the inner thighs or butt or stomach. This concentrates the pain on the most sensitive area, rather than spreading it out. This is why I use the nettle apron. PIC. and old POST. Working with one single stem of a few leaves can be more effective than a big bunch of stems and leaves.
There is a little more information, particularly on using a sunglasses pouch, and some video links in this post.