Top skiing starts with the bottoms. According to Doug Colley of the Ski Center, "Ninety per cent of the skiers could improve their skiing just by maintaining their equipment better. That means keep your petex bottoms repaired and waxed and keep your edges sharp."

Most skis need a pre-season tune-up, which can cost $10 to $20. If you ski often or carve up a lot of ice and rock, your bottoms might need attention several times a year. With a few hand tools and some practice you can do as good a job as a pro. Start by cleaning the bottoms of your skis thoroughly and follow the other steps the condition of your skis requires. Work where you can make a mess.

You'll need some cold-temperature wax (about $1), a metal or plactic scraper shaped to remove excess wax from the center groove (under $2) a 10 or 12-inch No. 10 bastard file (about $3) and a file brush to clean it (about $1.50), and a cork block (about $1). You may also want a surform blade ($2), but it's optional. 1. FILLING BOTTOM GOUGES

Set the ski, bottom up, between two stable objects, such as chairs. Clear the area of highly inflammable material. Petex burns very hot; any dripping petex should fall on cement, boards or material that is not highly inflammable. Don't work over newspaper, plastic or rags, and don't let melted or burning petex touch your skin.

Before melting any petex onto the ski you need a small blue flame at the tip of the petex candle. Light the tip of the candle, holding it so it doesn't drip on you. Rotate the candle with the flame almost touching a metal surface until you get a small, controlled blue flame free of black carbon flecks. You don't want a yellow flame, which causes carbon.

When the flame is small and blue, hold it 1/8-inch above the gouge and drip the melted petex into the gouge until it's full. When it's cool and hard, use a scrapper to level the surface, as shown in step 7.For major repairs you may need to use a surform blade first. 2. SHARPENING BOTTOM EDGES

Sharpen bottom and side edges except for the curve of the tip and 3 inches below the curve. Sharp tip edges will cause skis to "hook".

Hold the file at the angle shown, and keep your thumb pressure directly over the edges. File foot-long sections at a time until a clean sharp edge appears. Hold the straight edge of the scraper across the bottom of the ski to check for flatness. Concave bottoms will "catch an edge"; convex bottoms will be hard to set when turning. 3. SHARPENING SIDE EDGES

This is harder than doing the bottom edges. Don't let the file wobble; you want a 90-degree angle betweeen bottom and side edges. Hold the file lengthwise, applying pressure with your thumbs directly over the edge. Don't over-file; take only a few rhythmic foot-long strokes until a clean, sharp edge appears. Work gradually up the ski. 4. FINISHING THE EDGES

After filing the bottom and side edges, use a sharpening stone to remove any burns. 5. APPLYING WAX

Heat an old iron at a low setting so it will just melt wax. Drip wax along each side of the ski bottom as shown. Caution: Too hot an iron risks damaging the ski. 6. SPREADING WAX

Spread the wax evenly, with constantly moving strokes. Don't let the iron rest on one spot. Ski bottoms and interior fiberglass melt if overheated. 7. REMOVING EXCESS WAX

When the wax is completely spread over the ski bottom, let it cool and scrape off all excess. Scrape out the center groove. Leave only a thin film of wax, so the pores in the petex are sealed and the bottoms are waxy smooth. Rub with a cork block for a satin-smooth finish.