As you move upward on the design, the greenery will begin angling upwards as you fill in.
Clean off the ends so a bare stem inserts into the foam.
You can see how the greenery gets a little shorter and begins angling up.
The greenery in the center will go straight up and even eventually curve inwards towards the glass chimney so it conceals the green chimney pick.
Turn the greenery sometimes so the sides face upwards. This gives your design a better shape and keeps the centerpiece from looking flat.
Here I've drawn lines to show the general insertion curves of the greenery.
Every good designer knows the importance of concealing their mechanics, which in this case is the foam and chimney holder.
Here's an upper view from the end of the design.
You can see that it begins to fill in nicely. Spruce is thick and bushy and fills in nicely. Douglas fir does the same - it's just a little more of a green color.