Good video on propagating apple rootstocks by stooling.

Plant a rootstock tree that you want to propagate - I found I can buy rootstocks for just a couple of pounds, but when planting a whole lot of trees for an orchard you can save by doing this.

You can also do this if you have a failed tree on a known rootstock, for example if the grafted wood died out but the root stock survived.

Grow the root stock tree for a year or two

Cut the tree down to ground level (coppicing it effectively)

Put a large pot around the stool with the bottom cut off

Fill the pot with damp saw dust

Let the new stems grow up for a year and they will root in the the sawdust

Cut the stems low and plant out in desired location