The newest bouquet holders contain Oasis MaxLife foam which extends the life of flowers better than plain water. I always treat the water that the holder is soaked in with a flower food like Crystal Clear flower food.
Take care to mix the food and water carefully as directed on the label, Overusing the food won't hurt the flowers, but it is a waste of product. Shorting the amount needed isn't going to make much difference in the life of your flowers.
The first flowers inserted into the foam will build the frame of the bouquet, establishing the final size of the finished design. I begin by inserting a focal flower into the very center of the bouquet.
Clean off the bottom leaves and make a sharp diagonal cut on the stem of each flower. Insert the stem about an inch (deep enough to hold fast) but not so deep the the stems from each of the flowers start crowding the inner space of the head of the foam.
To create a balance, you can see from the sides the rounded curve that the insertion of the flowers are created. The stems are relatively the same length and I am working to maintain a balance and gently rounded shape.
Watch your arrangements both from the sides and the top to keep the insertions relatively the same and even.
A top view shows that the bouquet is maintaining a round shape as I continue to add flowers.