I started just gluing the largest ornaments I had on the wreath in any order (I am severely OCD, so I do red, green, red, green, you get the idea). I also place my letter on to get an idea of where it will go so I don't glue any balls on where it will be placed. Once they are on there you don't want to have to move them!
At this point just start gluing all different size and color ornaments until it is full and can hang on the wall without showing any of the green styrofoam. You will end up with about 3 layers of ornaments and they will also cover the sides. It will take a while to get it finished, so have patience! :-)
(BTW, this is last year's wreath! Sorry girls, but I still haven't got around to making this years)