Pinching Out or Disbudding Chrysanthemums
As soon as plants reach around 6-8in in late May or early June, pinch out the top growth, taking it down to three or four leaves up from the base of the plant. This promotes the formation of side shoots and you'll create a bushier, more floriferous plant.
If you want one large flower per stem, you will need to disbud the plants. As they grow, remove all the side shoots to create one strong stem which will produce one flower.
Chrysanthemums tend to snap at the base of the stems, so staking is really important. The stakes should be tall enough to accommodate the plant, but short enough for the flowers to be above of it; check the height of the varieties you have purchased.