When cleaning your flowers, don't discard the stems that you cut off while designing. Instead, you will want to rinse the stems off and then remove any thorns or excess foliage.
I do like to leave a few laterals of leaves at the very top, which will cluster beneath the underside of the bouquet holder, adding to the realistic look of being a hand tied design.
It would be wise to spray any leaves with a leaf polish to seal and hydrate them against wilting.
Be sure to use a mixture of all your stems, including the stock as well as roses, for the color variety and interest.
Keep the stems fairly long. You can cut the final stems to the desired length after you are finished attaching the stems to the bouquet handle.