I've mounted quite a few of my own bindings over the years, never used a jig. I used to just line up the boot center on desired ski position with the toe pieces snug, mark it. Mount the toes. I used squeeze clamps to hold them snug over the correct position, helps when they come with removable plates/risers that match the hole pattern. Then, line up the heel and mark the heels, mount then adjust forward pressure. I'm getting older and my eyesight and grip and work ethic is slipping so I have decided to make a paper template for the Salomon 800-900 series bindings I use most often. Have a pair of new skis on the way.
Take a ski that has a good mount for the same kind of bindings you will install on your new skis, remove the bindings, and center a pre cut heavy piece of paper lengthwise over the hole pattern. I used a file folder for paper.
Mark the center long ways and tape it on the ski centered over the holes.
Find your "good holes".
Put the binding back on over the paper. Put your boot in.
Mark the mid sole point for you boot.
Use a t-square to draw the mid-sole line on your template
Punch a couple of holes over your mid-sole line so you can see the mid sole +- lines on the new ski when lining up the template.
Try not to drill all the way through your new ski when using this. I make a small wooden block to cover most of the drill bit as a stop to prevent going too deep
Mounting bindings, what glue to use, etc is a different topic pretty well covered in these forums.
When you get the bindings on your new skis, take them to a shop and have then properly adjusted for DIN and forward pressure. Shops won't touch some older bindings though.
Or, just pay someone to do it that knows what they are doing!