You can spray the DE on tomato plants with something like a DustinMizer Power Duster if you have one:

Dustin Mizer

Or, if you have a puffer duster, you can use it:

Puffer Duster

Both of the above are more efficient than trying to scatter it on plant foliage by hand.

Some people put the DE in something like a knee-high stocking and tie a knot at the top and then gently hit the sock against their tomato cage or stakes to knock the dust out of the sock and onto the plant. You will not necessarily get real even coverage with this.

Some DE comes in a shaker can or with a puffer bottle cap that you can use to apply it, but the holes in the shaker cans are pretty large and I think a lot gets wasted. I've never used the DE that comes in its own puffer bottle.

Remember to wear a respirator mask so you do not inhale the DE. Remember also that rainfall reduces its effectiveness as it softens the sharp edges. I always wait and apply it after heavy rain when I use it, not ahead of heavy rainfall that will render it ineffective.

A great alternative to DE that is easier to apply is Surround WP, which is a kaolin clay wettable powder that can be mixed with water and sprayed onto the plants. Once it is dry, the fine layer of clay coats the plants similar to the way DE would.

To discourage cutworms and other pests that might approach from the ground and climb up the plant stalk, you can put a ring of DE around the plant's main stem.