Sorry, my mistake.
I'm pretty sure the plate has a center mark on it as does your boot. The toe piece on a Rossi demo binding has an "A,B,C" setting switch, right?
If your boot doesn't have a center mark, measure the sole of the boot in mm and find the dead center. Line that up with the center line on the plate. Now you know your boot is where it should be (providing you want it dead center).
Simply slide the toe piece to position "A,B or C", whichever allows the boot to remain centered. Then lock the lever down.
The heel piece, when parallel to the ski, then needs to be slid up to within a couple of mm of the boot sole. Just let the boot rest in the toe piece as if you were going to step into the binding. The boot should now be resting inside the toe piece and on the little "step" of the heel piece. Lock it in and you should be set. I know "a couple of mm" is vague but you get the idea.
Again, I would take it to a shop and have them do it just to be sure and to perform a release test. They might not even charge you because it will only take them about a minute to adjust a demo binding. Good luck.