The depth of insertion is going to determine the overall size of the design. For shorter pieces, insert close to the foam as shown.
If you want to cover more area quickly, use the advantage of the longer stem lengths to cover more area. This will take more greens that with the tighter, shorter stem approach.
Layer and overlap the fatsia leaves, poking a slit for each insertion with your knife. Cut a sharp point on the end of the stem at a slant for easy insertion into the foam.
I placed another stem at the bottom to determine the overall length of my flower piece.
Fill out the sides. For a more narrow arrangement, keep the sides in tight. For a more open, covering look, lengthen the stems to cover more area.
These Fatsia leaves are actually the SMALL size! You can see how the leaves will quickly fill in the piece, covering a large portion of an archway in a short amount of time.
I add another Fatsia leaf on the right side of the design.
Don't be too concerned about filling in very thick at this point. I will layer in other types of greenery, adding texture and different shapes to give interest to the design without relying only on flowers for the interest.