This will come naturally, but if you want to just grind it right away, the fastest method is to go to world 2 stage 5. In this stage you can get 64 bananas for every 15 second clear. You simply go down the ramp as normal, head right, and jump over the wall straight ahead to collect all 6 banana bunches. You should try to jump in the goal and pick to replay the level, as it's faster than quitting out. If you're bad at the game, you may find it nerve-wracking since the path has no rails, but it's just a straight shot to collect all the bunches, so you can definitely do it even at a novice skill level. This is definitely much faster than any of the bonus stages once you have it down.