Hydrangea paniculata, like 'Tardiva', lend themselves best of the hydrangea species to being grown as a standard. And they are seldom, if ever, grafted but grown on their own root stock.

It is best to start with a quite young plant that shows a tendency towards strong vertical growth. Select the most predominate stem and stake it. Remove all the lower stems and the shoots emerging from lower on the selected 'trunk', leaving only 3 or 4 of the uppermost shoots. Pinch out or cut back the tip of this stem to encourage additional branching at this point.

Training as a standard takes time - that's why pretrained specimens in nurseries tend to carry hefty price tags. You will want to attend to a similar process as the initial staking and shoot removal for several years, gradually increasing the height of the trunk by removing the lowest growing shoots/branches until you reach the desired height for your canopy. The taller the standard, the longer it will take to achieve.

You may want to check through the archives on the Hydrangea forum. There have been various posts discussing this trainiing technique over the years that might explain the process in more detail.