General Decorating Tips
I've used the word "string" throughout, but thread, twine, yarn, ribbon, dental floss, or wire also work. Select an undyed and preferably organic option if it will be in close contact with food, especially moist food, that you plan to eat. Use something biodegradable if you will be composting the ornaments after the Holidays. And choose a pesticide-free or organic tree if you can, since most commercial fresh trees are sprayed heavily with pesticides.
If you buy a conventional tree or use a faux tree (which may be high in lead) hang your edible ornaments so the food parts are not in continuous contact with the tree as much as possible. Lead is also present in Christmas tree light wiring, so keep any edible ornaments from touching those as well, or go light-free and decorate with strings of cranberries or popcorn, instead (your energy bill will thank you).