Ive done it lots of ways. In the early years I would run up to em like they do on tv and stand on his head or neck till he quit flopping. That way makes a mess out of how your turkey looks though!
The last thing you want to do is try to grab those legs! Instant cuts and spur gouges!
Last year one of my sons shot one and lost part of his pattern to foilage and saplings. He picked the bird up and as we were walking off I happened to look back at him and that turkey HAD HIS HEAD UP LOOKING AROUND! i told him to hold on tight and I picked up a stick about the size of a little league baseball bat. As the bird stuck his head out to see what was going on, I hit him right at the base of the neck as hard as I could! We killed that one with a big stick!
Generally speaking, nowadays I will get hold of his head right at the base of the neck and with BOTH hands, wring his neck. It is very important to not let his head slip or roll in your hand as you wring it. When you feel that neck snap in your hands yuo know that one is done for!