This will be unlocked upon completing Skate Park on THPS or Marseille on THPS 2 during a Skate Tour.
All you need to do? Come in 1st, 2nd or 3rd. You will need to do this to progress with your Tour progression as doing so unlocks other levels.
All you are trying to do is beat your rivals. You'll have 1 minute to score as high as possible, and three attempts. After each run, you will be judged, and each judge will give you a score of 0-99.99, and your score will be the average* of the five.
For each run you'll want to be getting 90+ to get gold, 85-90 to get silver, and 80+ to get bronze.
It's about 65k to get 85 points from the judges, at least on Skatepark.
Best way to rack up points is to use gaps between different areas. Manual if between things to keep your combo going. The higher the score the better your chances of beating your competitors, so keep those combos going!
*I'm not actually sure how they are calculating the score, as any way I combined the numbers, I never got what I score was, however average between the five was the closest number.