These grow tall with an open base on their own. However old ones I see seem to have a tendency to have leaned over at some point. Which doesn't look good with the upward orientation of their branches.

How is it done?

Let the plant grow tall. Instead of cutting off the tops, Limb it Up. Remove the lower side branches from the bottom and remove any 'trunks you don't want. It will be happier as a multi-trunked small tree.

it takes 1-3 years to train into standards with good form. You must first let the plant grow tall over the season, limb the start with only the top 3rd of the plant with leaves and branches until desires high, then pinch the top and allow to branch, prune each branch back to 4-6 buds and let grow out over and over to form. Here are mine just budding out this year- cuttings of Diana grown into standards :-)

The one in the back is only 1 year old, the other three are now 3 years old.