I just got a few pairs of Atomic skis, both GS and SL and noticed they came from the factory set at 1 degree base bevel and 2 degrees side. Since I already moved my son to a 3 degree side bevel, I had to increase it for the new skis as well... so here's how.
To find the current bevels the skis are at, use a thick dark marker and mark the edge, about a 1cm/1inch strip every two-three inches or so. Then get an angled bevel guide with a medium 200-400 stone and work it. Change the angle guide until the markings are uniformly wearing off and that's the current bevel.
If the current angle is higher than what you need, say for a pair you just got second hand, the same sequence below applies, except you don't need to file as much. Likewise if you just got new skis and need to sharpen them well - so in reality, you can skip this investigative step and just get on with filing the skis :)
In terms of tools: a rough (body) file, a medium file and a smooth file, diamond stones 200-400, a few side bevel guides (to find the original angle) and a base bevel guide matching the ski (for de-burring).
First, make sure the base is flat enough for your purposes. For a U14 racer or myself, I accept some minor normal variations from the factory, for a U16 and higher, you may not. Depending on how severe that is, it may mean a base grind or just you messing with sandpaper for a bit. If the base is not decently flat, you cannot have a decently consistent bevel on either the side or the base...
Should I mention you should make sure the base and edges are clean of wax before you start or is it obvious they will gum up your tools if they're not?
Then, you need to protect the ski base from all the metal bits and filings you are about to produce. I prefer to fill the base with soft wax from hot-scraping it once, then clean it up really well - this will make it harder for the metal bits to find any room to embed into.
Most technicians prefer to cover the base with packaging tape - you should give that a try.
Make sure the sidewall is removed enough, especially for new skis. I use a 6 degree guide with a panzer file. Then tape the sidewall with painter's masking tape, to keep metal filings from getting into the bindings.
Mark the edge with marker again, this time more frequent: 1 inch strips every two inches. You will find that the skis come from the factory with a pretty inconsistent edge, which makes it very hard to sharpen them - you need to smooth out the entire edge uniformly and that will mean filing more in some areas.
Set the ski on a side, set the rough file on the guide and get on with filing the edge. Make short overlapping strokes, cleaning the base and file every 2-three strokes, before moving on. Keep a clean paper towel or rag in the other hand and wipe the base often. Blow or brush off the filings from the file as well.
Keep checking your base to make sure you're not leaving deep gouges from some metal filings you trapped between the guide and the base.
Use light pressure on the files, let their sharp teeth do the work. As everywhere, more light passes are better than fewer heavy passes. After about 3 end to end passes, the marks should be wearing off uniformly and almost gone - you can switch to the medium and then the fine file, two light end to end passes each.
Then, clean the skis and flip them on their base, take the base guide with a 200-400 diamond stone and go for 2 light passes, to remove the burrs left by the files.
Flip the skis back on the side and use the 200 diamnond stone and then the 400: many passes with each of those, to finish the edge. If you're close to race season, go up to a 600 and a 1000 as well.
When done, use the steel and brass cleaning brushes vigorously, then hot scrape the base a few times. Use a brush and brush out the sidewall, bindings etc, remove all the metal filings.
The biggest issues here are protecting the base from the metal filings and making sure the resulting bevels are uniform. Mark the edge every time: don't rely on your eyeing the quality of the edge directly.
I prefer to use the files on both sides of one ski and then the diamond stones, since the diamond stones need watering. If you have two guides, you can put them to good use here and save having to flip the ski that much.
The more you clean the files and skis, the better your chances of getting a good result.
Guides that glide on rollers are a better choice because there is less contact with the base and less chances of scratchign the base with some metal filings.
While filing, use light pressure, pay attention to the direction the file works better and pay 110% attention - a mistake here may cost you many hours of work to remove any skid marks you leave behind or worse, to restore a damaged edge. Also, while filing, always pay attention to the action of the file: it will skip over some sections and will bite more in others - you are looking for a consistent edge.
Brush out the metal filings from the sidewall when changing the files.
If you haven't filed a lot of skis recently, practice this on some crappy pair of skis first.
Read on for even more Ski Tuning Ideas.
Good luck and have fun!