New skis will be dirty from the manufacturing process and have a layer of factory wax.
NOTE: Never use base cleaners on your base. Always clean by the hot-wax-scrape method. It is good for your base and repeated waxing keeps the bases running fast.
Scrape the factory wax off with a plexiglass scraper.
Use the hot-wax-scrape method to completely clean the ski.
Choose an inexpensive soft (warm-temperature) wax. The red (warm) hydrocarbon wax sold here is excellent for this purpose.
Set your skis in a vise and hold the brakes back with a brake retainer.
DrD recommends a Ski-Man vise; they are rugged and will last a lifetime.
Iron on a coat of wax and scrape it off while it is still liquid. You may see dirt or grime in the hot wax.
Repeat waxing/scraping until the wax is clean.
If you use someone else's cheap quality wax for this purpose, try to select a better quality wax for the final wax/scrape so the last thing on your base is a quality warm or cold hydrocarbon wax. If the ski conditions are cold, start with a warm wax to clean, but switch to a cold at the end.
Allow to cool at room temperature.
Follow up with a brass brush; leave as is if you are done for the day.
Check for flatness
Hold a true bar to the ski at 8-10 points along its length (see picture below).
You are looking for a base that is consistent along its length, neither high nor low compared to the edges.
Base material should be free of waves.
Add structure to your base
The average skier may wish to retain the factory grind; a racer should consult with a shop technician that knows what grind is performing well in your region.
For a new ski being stone ground by a technician, consult with him/her to decide if light passes may be sufficient.