Use an artificial water kit or a clear polyester casting resin to make the imitation water. Instructions for each of these kits vary. Follow the directions for your specific kit.
Make a grid with floral tape going from one edge of the vase to the opposite edge. This will help your flowers stand up in the vase while the water dries. However, if your arrangement is full enough to stand on its own, skip this step.
Pour the imitation water into your vase. Be careful not to splash or spill the resin on the sides of the vase.
Arrange your flowers in the vase. Let the artificial water dry according to its package directions. Remove floral tape if it was used.