I think the word massage is a bit misleading - I like to cal it perineal stretching.
The word massage gives the message it's all soft focus lighting and gentle touch etc whereas if perineal stretching is done right it should really be at the limit of comfort and beyond.
You are stretching the vaginal muscles inside the vagina in order that they do not tear during the actual birth of a baby.
The baby's head is bigger than a thumb so who ever is helping you out really needs to lean on it a bit, and stretch stretch those muscle bands inside the vagina.
Get a physiotherapist or midwife to show you and your DP how, and especially if you have had a tear before, and do it as early as possible - 24 weeks is NOT too early.
Scar tissue is less flexible than normal tissue, but with pressure it will stretch - so get started on it asap.
The midwife at the birth should really be hands on stretching you out at the birth, with hot towels and lube / or oils. You can write this in your birth plan that she helps you out in this way.
A note of caution: oils / and essential oils can sometimes cause thrush and other nasties, so use a KY lube if needs be - or nothing at all if you're OK with that.
But the important thing is to do it early and often, and to do it till it's uncomfortable and then some.