The short answer is, meticulously.
The process is time consuming, but fairly straightforward.
Ski shops have tuning machines, and don't charge much, so most skiers prefer to just drop off their skis at the end of a day and pick them up ready to go in the morning.
For those who prefer to do it themselves, here's a rundown of the process.
Set the skis on a workbench, base up, held in place by clamps of some type.
Wipe down the base with a clean, dry cloth.
Brush the base with a stiff brush. Some people use wire, others consider this anathema and use only nylon bristles.
Melt on cleaning wax by holding a stick of wax to a warm - but not too hot - iron, over the skis, and dripping a line of wax drops from one end of the ski to the other.
Iron the drops into a smooth coating. Then scrape it off with a plastic scraper. This wax was only to clean the bases by picking up every speck of dirt, oil, or grease.
Then brush the base again with the stiff brush.
Then melt on another coating of running wax, suitable for the temperature of the season.
If the skis are to be stored, leave this coat of wax as is. If they are to be used right away, grab the scraper and scrape most of the wax off, and brush the base with the stiff brush.
Check the steel edges, and if they are dull, or have nicks, run a sharpening stone along them.
Frequency of waxing depends on how often the skis are used, and the conditions they are used in. Some people go an entire season on one waxing; others wax before every race. There's a wide range in between. Check the base every time you ski. If it starts to look dry, it's probably time to wax.
Question: How do you tune skis?
Here's a video of amateur racer, and former freestyle skier, Shaun Sutner tuning his skis.