Actually, my gynae in Spain sends her patients to a physio who specialises in it. To apply the pressure in the correct place and release the knots in the muscle in more than just the vulva it has to be done by someone else. The perineum is a set of huge muscles which go in deep between the anus and vagina, it isn't just the bit of skin between the vagina and anal sphincter.
The physio taught DH how to do it too, though she was much better, but to do it properly involves the person doing it to:
1) Lube!
2) Insert first one, then after a few goes two fingers up to about the second knuckle facing your bottom.
3) Then they move from one side of the vaginal wall, along the back of it and up the other, applying pressure - hard pressure - and looking for lumps.
4) Press hard on the lumps (this should hurt a bit) with the finger tip for 30-60s until the knot releases a bit (the pain then goes too) and move along to the next point.
5) Repeat with fingers slightly further out.
Before going to the physio I tried myself and it is physically impossible.
I did not have the best birth experience as daughter was born tangled in her cord with her arm against her head and it was only due to several weeks of this preparation that my perineum was elastic enough to cope with what the midwife had to do to get her out - she actually said my perineum was amazing - I felt quite proud!
There is a lot of medical evidence to show it works when done properly, after all, like any muscle in your body it becomes stiffer and stiffer with age - I can't still do the splits like I could as a kid!
Here is just one extract from the British medical journal in support.
https://onlinelibrary.wiley.com/doi/10.1111/j.1471-0528.1997.tb12021.x/abstract
I don't have to do it for a second child since the first birth typically leaves it stretchy enough.