Growing rootstock?

You'd have to start with a bought one, then coppice it and mound it up a bit like a mole hill, all the resulting shoots will then produce side roots into the mound and in the following winter you can remove the mound, a hose or pressure washer is good for this. You then have rooted shoots that can be clipped off the parent plant. Thus they are all clones of the parent.

Although to be honest I do n't think it's worth the effort you can buy rootstock for between £1 and £2 each, you know exactly what you are getting and it saves all that hassle. I really do n't know why they say you are best growing your own.

I have grafted most of my own trees on bought in M26 and MM106 rootstocks both of which make ideal garden apple trees, in terms of size spread and managability.

I'd just buy some in and get grafting, it's a great skill to have and lots of fun creating your own fruit trees.

Buy-in the rootstock you need.
Grow it up for a few years.
Then coppice and layer to produce a stoolbed.
Considering the time lag, you'd be just as well to buy-in "clean" rootstock of the type you need; they can be had for as little as £1-2 each.

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But 1st year layered rootstock are tiny sticks that are best planted to grow-on for one season and bud-grafted in late summer.
A year after planting, the rootstock top growth is removed and the grafted bud allowed to sprout. It will take another 3-4 years before you have a small tree that is branched and ready to fruit.

Alternatively, buy a Bramley on the rootstock that you want, grow it for a season, then hack off the Bramley bit to leave just the rootstock.
I suggest Bramley because it's such a ferocious grower that it'll turbocharge even a dwarf rootstock; a Bramley can grow up to twice as fast as some varieties and about 50% faster than an average variety.

Alternatively, but a cheap tree and re-graft the main branches to whichever variety takes your fancy.

Any tree or rootstock which remains in one place for a long time will eventually get diseases, so the quality will gradually deteriorate.

It is also possible to use apple seedlings (from pips). However, most shop-bought apple seedlings have no disease resistance and therefore die within a couple of years.
Any seedlings that don't get diseased can be very variable - some will behave like vigorous rootstocks while others will behave like dwarfing rootstocks. They are likely to be less productive (slower to fruit and less fruit per tree) than the regular apple rootstocks which were specially selected for being excellent rootstocks.