For this fall centerpiece I just choose flowers randomly, selecting the ones needed for my monochromatic (all same shades and hues) color scheme. This is actually an artificial baby's breath in bloom.
I cut apart the bush and wired the smaller heads so each would have a stem to insert into the foam.
I look for "empty holes" for places to insert my flower heads.
As you can still see some of the foam, these are the spots you want to fill in as you go.
Turn your arrangement around as you go, making sure to balance the entire arrangement if intended to be a centerpiece for the middle of a table
I inserted some of my baby's breath to extend up and beyond the lower heads. Depth is important.