Are you sure you know what you're doing...cause it kinda sounds like you don't....
You want to mount the center of your boot to the line where you want to mount your skis, there is some sort of marking on the bottom of your boot to identify the mid-sole (I know on Full Tilts it's an arrow, others it's usually just a raised or recessed line). As far as where to mount them on the ski, measure, if you want true center take out a tape measure, measure the whole length of the ski (it doesn't matter if you lay the tape measure flat through the curves of the tip and tail or if you let it float as long as you're consistent), divide that measurement by 2, measure out to that number, make a mark (I usually put a piece of masking tape down around center then remeasure and draw my line with a sharpie on the masking tape), repeat for ski number 2. If you've done it right you can put the tip of one ski against the wall and the tail of the other ski against the same wall and the lines will match up (or at least very close). Then mount based on your boot sole length to that line (obviously make sure you have both tips facing the right way again).