The first thing that I did was remove all the old foliage and then I put the center ball into place on the stems and glued it into place.
I would suggest that you make a hole through the center of the styrofoam balls with the dowel rod so the hole is in place but then remove the ball to apply the boxwood as it's much easier to work with detached.
The boxwood that I purchased has stems attached to a branch so I removed all the stems first and then I cut the stems to a size that was appropriate for my topiary. I was able to get two boxwood pieces from each stem.
Then I stuck the ends of each piece into the styrofoam ball, working around until it was completely covered. I found that I didn't have to have them right next to each other to fill it all in. I used (6) packages of six to cover all four balls.
Once the balls are covered, you can put them on the stem or dowel and glue them into place.
Even though mine are in dirt from being real plants, I'm going to remove the dirt and secure them into place with plaster of Paris
Once set, I will put my topiaries into decorative terracotta pots.