Nothing adds a bit of Christmas cheer to a house like a wreath. The ones that you can buy, though, are often over-priced and, to be honest, not that great looking. Making your own wreath, though, is a great way to get into the Christmas spirit.
To start with, you need a base to hold your wreath in shape and give it structure. I have a woven wooden wreath that I bought for a few pounds at a craft store, but you could also use a floral foam ring, a shaped wire, etc. You just want a basis to work from to help hold everything together and keep it in a circle.
Then you can use whatever greenery you have available. My daughters and I went into the garden with a basket, and picked some holly, evergreen branches, rosemary, roses (including leaves, rose hips, and flowers), St. John's wort, ivy, etc. Pick a nice selection of green leaves from whatever you have growing around your home.
Then shape it around your ring. My base ring is woven twigs, so I can stick branches into it, and you can also use floral rings to push things directly into. However, you could also use twine, string, or wire to secure the foliage to the wreath. Experiment moving pieces of foliage around the ring until you find a shape that you're happy with, and add some "features": rose hips, holly berries, a twig of mistletoe, etc. If you want, you can add a nice bow out of ribbon, some cinnamon sticks, or other Christmas-y items.
When we finished the wreath and hung it on the front door, Miriam exclaimed, "Wow. The house looks all beautiful for Christmas. Good job mummy!" Then she insisted that we use the leftover greenery to decorate inside the house, so we now have beautiful leaves decorating our mantelpiece as well.