You can see more instructions and photos in my Fall wreath tutorial, but basically you lay out the flowers in the pattern you want. Then use hot glue on what is left of the flower stem and shove it in between the vines of the wreath until secure. I actually used an industrial glue gun which worked well because it got so hot, but a normal gun will work. And you might not burn a giant hole in your wrist like I did.
After you have the flowers on, lay out the leaves behind the flowers on the inside and outside of the wreath. Use the hot glue to place them as well. Be liberal with the glue and clean up the strings that get left.
Try hanging it up on the door and add an extra leave or two if needed- otherwise you are done! I considered adding a Christmas bow as well for a little extra cheer. That would look great too!