Prep your roses by measuring how long you want them to be and trimming them all about equally. You want them about 4-5" longer than the vase when positioned diagonally. Remove any leaves that will fall below the base of the vase.
Fill your vase with water. Add roses to the vase allowing them to fall to each side of the vase {see video}.
Start filling in {alternating a few of another color if you choose} using the existing stems to help you position new roses making some vertical.
Trim eucalyptus to be an additional 6+" longer than the roses and again remove any leaves that would fall below the base of the vase.
Insert eucalyptus around the edge of the vase all the way around or at 3 opposite points.
Add small stems of eucalyptus {3} nestled in between roses at opposite points in the flower arrangement and adjust roses as needed to appear full.
Notes
roses open with time - this centerpiece because more beautiful as the days pass, just refresh the water if you don't like how it looks, adjust. small tweaks can make a big difference