Probably the first part of the process is to put in a sturdy stake and tie your 'main stem' to it using plant ties that you can easily loosen as the plant develops its trunk.

Decide how tall you want the trunk to be.

It is easiest to prune or train the plant when the new shoots are very young; just gently rub them off.

Season by season pinch back the flowering part of the plant so it doesn't develop long and groping stems. (Similar to what you would do to get a good flower crop from Fuchsias.)

Each year tie in the central stem that's forming your trunk, raise the crown a little by pruning off the lower branches to leave at least a third of the plant as leaves, stems and flowering wood.

Protect it all from strong winds.

Remember to check the ties each six months to be sure that they are not starting to cut into the developing trunk.

As the plant gets higher allow the side branches which will form your spreading display to develop.

Information on developing Wisteria, climbing roses, honeysuckle as a standard with a spread flowering system is likely to give you more detail you can use specifically for your plant.