What have you tried so far?
Once you get it once, it is extremely easy to find it again by creating a signature for the instruction(s) that write to the address.
What I did was go in my house, scan for an unknown initial value (float). Move my character, scan for change. Then do other stuff in game like moving the camera and opening options, but keeping the player position the same and scanning for unchanged. Then move the player, scan for changed. Repeat this process. Once you have around 100 results, you can select maybe 20 at a time and change the value of all of them. If your player moves, then you can remove the other unselected addresses since its obviously not them.
You could also use another trainer to help (kinda cheating tho :P)