Our apple tree wasn't doing much of anything (it barely even ever flowered), so I finally cut it down and bark grafted 3 different varieties onto it - Liberty, Ashmead, and Cox Orange Pippin. These should all be able to pollinate one another. We also have a Honeycrisp tree nearby that should benefit from this, too.
Not difficult if it's sharp. I use a grafting knife, but a pocket knife works just as well.
Bark grafting may pose a problem since you'd be cutting into the bark of the trunk, but it's easier to do in the spring.
You may be other tools (like a saw) in order to do certain types of grafts.
You can get the angles to match up pretty well by hand? I guess you could probably practice on random twigs.
Yep. You just kind of eyeball it. I start with the branch I want to graft onto first, then cut the scion and compare it to the branch and just make adjustments as needed.