I'm filling out this design with just white roses. I usually wash my hands and spray them with Finishing Touch so that the oils on my fingers don't brown the petals.
After the first two roses in the center, I begin adding the flowers on the outer edges of the bouquet.
I insert the roses by holding my fingers around the stem close to the foam and working the stem gently into the foam. This helps prevent pushing on the head of the rose which can sometimes damage it.
I begin seeing "empty holes" and planning out where to place my next rose.