I'm also going to add some smaller sized pine cones. These are already pre-wired, being cut apart from some of the Christmas picks.
You can easily wire your own small cones if yours don't have a stem.
Simply wrap some florist wire (around a 21 or 22 gauge) around the bottom bracts and twist down tightly to form a stem. You then wrap florist tape around that stem.
Tuck the pine cones here and there all around the centerpiece.
Push them deep enough into the florist foam so that the extended wire doesn't show.
Don't leave them stuck to far out or they'll look like a strange afterthought to the design.
Instead, nestle them deeply in among the greens so they look like a natural part of the design.
Always turn your arrangement around as you add different elements so that the final look isn't lopsided.