If I understood the tuning files correctly:

- Each game has an independent, hidden skill that goes from 0 to 90. It goes up (0.09 points per sim minute) when playing or streaming the game with any device (mobile included), increases faster (0.125/m) when taking part in a tournament, and decays if not used (-0.0125/m).

- The sim gets a score based on the Video Game skill and the hidden skill in the game: 3 points for each 10 points in the hidden skill beyond 10, up to 80 (so 3 points at 20, 6 at 30, and so on), and about 2 points per video gaming skill point above 2 (so 2 points at 3, 4 points at 4, and so on). Career doesn't seem to have an effect, but mood (focused, confident, happy) do due to increasing the effective skill level.

- The place is random, with the chances based on the score the sim got; a score of 6 or less guarantees a failure (edit: 10 or less for Pro tournaments), and a score of 38 or more gives the maximum chance of winning. A score of 38 requires 80 or more (out of 90) in the hidden game skill and a video gaming skill that has been boosted by emotion or moodlet beyond 10 (happy and confident boost it by 1; elated, fearless, and focused boost it by 2; in the zone boosts it by 3).

So, basically, if you want to maximize the chances of winning: focus on a single game to get the hidden skill in that game up, practice about three hours per day to keep the hidden skill high (taking part in tournaments counts as practice), play for an hour or two before each tournament in order to get the hidden skill near its max, and get into an emotion that boosts the video gaming skill. Do keep in mind that winning is random; with a video gaming skill of 6 (or 4 when focused) and no practice it should already be possible to win a tournament, though it will be very rare.