Once the e-track is cut, and the rubber is cut and matched up under the e-track its time to drill. First ensure that the e-track is straight and in the exact area you would like. Then I recommend drilling one hole, slipping a bolt in it to hold it, then do the exact same thing on the oposite side of the etrack. Doing this will help ensure that the etrack won't move on you, then you can focus on drilling all the holes you would like. A couple things to keep in mind:
1. Ensure you get a decent amount of your holes through the frame or something else strong and enforced.
2. While you should ensure you use enough mounting holes on the etrack to secure it, you do not need to use every hole. That would be overkill (at least on standard e-track like the one we mentioned using above).