[mtownsend]

So, whether I use Kevlar, steel or Dacron guy wire, how should I tie off this stuff?

I typically use a figure-eight loop knot to attach the line to the various end-points (ring anchors, guy-wire ring, etc.). For tying into closed rings, you need to first put a loose figure eight knot in the line, then pass the loose end of the line through the ring, and finally follow through the first figure eight knot backwards with the loose end (a video might help). After you've learned this knot once, it's relatively easy to remember, it's very secure (never comes undone on its own and never slips), and it's a type of knot that doesn't put very sharp kinks in the line. There are other great knots that will work and might not require as much slack as the figure-eight loop, but I find this knot to be one of the easier ones to remember.

In most circumstances, you will need turnbuckles to apply the final tension tightening on the lines.

I hear Kevlar will snap when it is bent.

Kevlar does lose strength when folded or bent, but for mast stabilization, you're only talking about forces that are typically well under 100 pounds. Kevlar inside of a plastic sleeve will reduce the sharpness of the kinks, and a smart choice in knots will minimize the damage too. Hopefully, your mast will not be subject to stress levels that will test these limits.

Steel guy wire - does that twist on itself like the ends of a wire coat hanger or would I use electrical type of crimp connections?

Wire twisting will not work. It will loosen too easily and will not hold the required tension.

Crimp on fasteners work well, but then you need to buy the crimp fittings and crimp tool to do the job.

The figure-eight (or any other suitable knot) method also works, and does not require any extra tooling, however, it does require more slack at each end to allow you to tie the knot. If this kind of knot is new to you, then you might want to practice a few times in your living room until you've memorized it and also become better at judging how much slack you need at each end to complete the knot.