Ingredients you'll need
One 10-12 pound turkey
Salt and pepper
Dried spices like thyme, celery seed, or sage
Olive oil and/or butter
Preheated 450 degree oven
You can make your turkey and its flavorings more elaborate, but honestly a little oil and salt and pepper is all you need. I brined my turkey in salt water the day before cooking it, but you don't have to. I don't recommend cooking massively large turkeys unless you actually have to feed 25 people. a 10-12 pound turkey is more than enough for most gatherings.
Step 1: Cut Out the Backbone
You're going to cut out the backbone of the turkey. De-spine it, essentially. I strongly recommend using cooking shears if you have them, but a sharp knife will also work. You are kind of going to have to wrestle with this turkey, so give yourself room, have a towel or two on hand, and take your time. Bleeding on a Thanksgiving turkey is bad form.
Put the turkey breast side down and cut along both sides of the backbone. There are a few spots where this will be a little tough, so take your time and work with the shears in short clipping bursts. If you have gloves, feel free to use them to stabilize the bird if you need to. Once that backbone is out, toss it into your stock pot if you are doing that. Pry open your bird so that it's mostly flat while upside down.
Step 2: Flatten the Bird
Step 3: Season and Cook
The way I often do this is to just turn the oven off after an hour and let it sit in there for 15 minutes while the oven cools down. Your bird won't be all pretty and stand-up like the proverbial Thanksgiving turkey, but it's much easier to separate the thighs and legs and other pieces when you spatchcock it. I served mine with these wonderful Stuffin Muffins from an Instructable I stumbled upon and some giblet gravy. Turned out great, and it was way less time and stress doing it spatchcock style.