First off, get yourself a book and the proper tools, and ask yourself this question: "do I have basic mechanical abilities? or do I usually screw things up when I try to fix things?" I your the later, take them in to th shop and forgetaboutit, except for deburring and waxing - which my 90 year old grandmother could do. But to answer your questions:
P-Tex candle: Kinda hard to screw up your skis if you do it correctly, unless you touch the tourch flame on you base
: But no need to use p-tex candle anymore, they have iron on p-tex patches now.
Sanding: [img]redface.gif[/img] do not, I repeat, do not sand any part of your ski. The word sanding and skis should not be used in the same sentence, unless your makeing wooden skis [img]tongue.gif[/img] If your referring to the archaic process of belt sanding to flatten the bases, don't do it! Get yourself a base flattener or base plane.
How could you screw up your skis with a base plane? taking too much off, uneven pressure to one side, skipping and chewing up the base or edge. But don't be too scared, just pratice a little on an old pair until you get the hang of it. It takes a long time to take any significant base material off with it, so any one stroke is not going to screw up anything. Check often with true bar.
Waxing: hard to screw up here if you keep the iron moving and not too hot. If your wax is smoking like crazy, your iron is too hot.
Sharpening edges: You can screw up big time here if you bevel your edges wrong. For example, if you beveled your base edge to 3 degrees, then said oops; you would need to take a lot of base down to fix it.