Take the width of the door, and cut a strip of fabric this length by 8" wide. Cut two strips.
Cut out 6 or 7 triangles of green fabric, they don't have to be perfect! Use repositionable fabric adhesive to secure them in place, Crafters Companion Stick and Spray is good.
Add little pots underneath each tree, again, random patterns and sizes work well.
Using the free motion embroidery foot on your machine, sew around each tree three or four times, taking the stitches down to the pots.
And here's another idea for the tree!