I'm going to use blue spruce to flesh out my centerpiece and give it the initial shape of the design. Douglas fir also works well for this.
Clean off the bottom of the spruce and insert it into the two longer ends of the flower design.
Note that I angle the greenery upward as I insert it into the foam. This is so the greenery has a natural angle downward, lightly brushing the table top with the spruce tips.
Keep in mind that the length and width of the final design is going to be determined by length of the greenery. If you need a larger centerpiece, simply cut a longer stem, lengthening the greenery.
When flowering the design later, I will extend my flowers just a bit beyond the edges of the greenery.