You can attempt to jump onto a larger creature by making a Grappling attack. Alternatively, you can use your action to make a [[Strength](Athletics) or Dexterity(Acrobatics) check contested by the targets Dexterity(Acrobatics). If you win the contest, you successfully move into the target's space, clinging to its body. If the target loses the check by 5 or more points, then you have advantage on attack rolls against it until it makes a further attempt to dislodge you.
While clinging to the target, you move with it. If the target is at least two sizes larger, then on your turn, you can move around within the larger creature's space, treating the space as difficult terrain. The larger creature's ability to attack you depends on your location and is left to the DM's discretion.
The target can use its action to attempt to dislodge you–knocking you off, scraping you against a wall, or grabbing and throwing you–by making a Strength(Athletics) or Dexterity(Acrobatics) check contested by your Strength(Athletics) or Dexterity(Acrobatics) check.