Use some sort of paint program to do it. I recommend the GIMP or Paint.net since they're free. Open both of the files and then just use your paint skills to kinda put parts of one on the other, like the head of one or the clothes of the other. Then make fine detail adjustments like matching the skin tone and whatever. Just make sure you save it as a .png file when you're done.
Private Mod Note
Rollback Post to RevisionRollBack
Don't like the terrain? Go build your own. You can totally do that in this game.