ok so if you're going to do it at home, this is how you do it:
make sure you have your skis and bindings ready. now measure a line at the center. if it is premarked, good! if not, measure both ways (probably tip to tip) to make sure you have it center. If that seems weird, don't worry.
After that, grab your skis and bindings and boots and bring them to a shop and get them mounted. Specify center mount. And you're done!
Otherwise, you'll fuck em up.