Floral designing is all about attention to details. Notice how the edges of the foam extend beyond the pillar candle stake. If it didn't, you would have a hard time inserting flowers straight up they way they need to be in order to conceal the stake.
If your pillar candle is very tall, you may want to have extra security by taping the holder into place with waterproof florist tape.
I'm going to green in this piece starting with a standard fern used by every flower shop. Leather leaf fern has a lot of laterals that can be cut off by the piece for use in the centerpiece.
Be sure to cut with a sharp knife rather than tearing off the middle stem. This sharp cut is easier to insert into the flower foam.
I'm cutting off some single laterals so I can use them around the base of the pillar candle.
Note how I inserted the fern frond deep into the foam at a slight angle INWARDS towards the candle.
Continue doing this around the base of the candle, effectively concealing the plastic holder.
Now you can begin adding more pieces of the leather leaf, working from inside to the outer edge.