I've been looking for information about this too and can't find reliable information on it either. I find the previous comment helpful :)
My expert midwife sells an oil and webinar. They also have an article for how to with a YouTube video. I haven't started yet as NHS says from 35 weeks: https://myexpertmidwife.com/blogs/my-expert-midwife/why-you-should-do-perineal-massage-and-how-to-do-it
I've been told by my midwife to start only at 36 weeks, I've bought an aniball this time as I had an episiotomy with my first and want to make sure the area around the scar tissue is nice and supple. I did it with fingers last time but not sure if it was effective. The episiotomy was due to her being chin and rather than chin to chest and getting stuck and not because I was tearing or not enough space
i’ve jsut started this today as i’m 32+5, i’ve been told: Apply a natural oil to either your perineum or your thumbs. Place a thumb or finger about 5 cm (2 inches) into your vagina, and use it to gently stretch the wall out to the side. Call this 3 o'clock. Still pressing and stretching outward, sweep down to 6 o'clock and over to 9 o'clock. Repeat for a total of 5-10 minutes