Do you have a vice, or can you borrow one that clamps to a table? Wrap the boot upper in a towel (so it isn't indented), and tighten the vice on it to hold it still. BTW, It is easiest to make the screw perpendicular to the bottom (which the person in the first video doesn't do particularly well) if you clamp it so that the bottom of the boot is approximately horizontal. I have played with using a drill guide to force screw perpendicularity, but admit I didn't do that last time I mounted my blades - which meant the screws didn't lock the plate in place as well as they should have.
I hope that your blades have a pair of slotted holes in the back as well as the front - makes it much easier to adjust front and rear side-to-side 0offsets. Remember that the way your feet and boot are shaped may cause them not to center on the front and back of the sole, so you want to adjust front and rear offset so you can glide on one foot without a tendency to turn.
In the second video, the tech holds the boot in his hand while he drills, he risks injuring himself with the drill. Also, when you pre-drill - if you can remove the insole first, do, so you don't drill through the insole.
Also a lot of people don't want the boot centered the way he initially centers it. They prefer a significantly offset blades, to improve balance. Though as I mentioned, I personally prefer to put it approximately in the centerline of my foot, not the skate, and reshape an insole, or make my own, to alter balance and create full even contact.