Fit the floral foam into the container. The floral foam should be cut so that it fits into the entire opening of the container. If necessary, use several pieces of floral foam and combine them together to be wedged into the container for a tight fit.
Glue the floral foam pieces securely into the container so that they will not come loose after you have determined which pieces are necessary for a tight fit.
Cover the visible top of the floral foam with a layer of floral moss. Tuck the moss in along all of the edges of the floral foam securely. It is not necessary to use any hardware to attach the moss to the floral foam.
Begin to cut the stems of the hydrangea blossoms to the proper height for your container. The height of the stems should be approximately two times the height of the arrangement container. Vary the heights of the stems by an inch or two so that they are not all the exact same height.
Insert the stems approximately 1/2 inch apart into the floral foam. Experiment with the placement of the stems until you find an arrangement that you like.
Add some filler greenery in and around the hydrangea blossoms to fill in the arrangement.
Step back and look at the arrangement. Make any adjustments necessary until you are satisfied with the result.