I'm part way through a similar
project. I bought a single M25
apple rootstock last winter which has had 12 months in the ground now. It was planted with a large plastic collar filled with woodchips - a ring cut out of an old plastic drum. Over the weekend I lifted the ring off, pulled the chips apart, and the stem had put out beautiful
roots.
I snipped the stem at the soil level, put the ring back in place, and refilled with a barrow of fresh woodchips. My expectation is that by sprint the stool will sprout a few dozen new stems which will grow their own root systems and I will be self-sufficient in rootstocks for my future grafting projects.
The original parent stem is now in a pot and will get a graft of an eating variety in a month or so.
I have chosen to do it this way because we have shallow soil on top of chalk and the M25 has a reputation for growing to a large size under conditions which would stunt most other apple rootstocks. I want to gradually replace the weaker trees that we have around the place with ones that need less babying.