There are two different ways to use empty paint cans as planters: as solid bottom vessels for cut flowers and as sturdy planters with drainage for greenery. If the paint cans are going to be used for greenery, you'll need to add proper drainage by drilling holes in the bottom of the cans.
Before starting to add the dripped paint effect to each can, remove any labels and protect your work surface with kraft paper. The paper will catch the dripped paint, but you'll need to remove it just before the buildup dries to the point of hardening.
An easy and effective way to apply paint in a dripped manner is with a turkey baster. To do this, insert the turkey baster into the paint, then squeeze the rubber top until 1/3 of the baster is full. Next, add paint to the exterior of the can by slowly releasing it from the turkey baster.
In order to truly showcase the updated paint cans as flower vessels, it's best to stick with one type or color of flower, then group the various sizes together. When several colors or species of flowers are used, the variation can detract from the vessels.