Okay, so to get another Star on your Save File you need three crowns on your extra life count, this means reaching the maximum number of lives, 1110.
First off, you will want to have both a Super Leaf on you and a Super Leaf in your reserve. Tip: Once you complete the game, Mushroom Houses become permanent.
Next, head to World 2-4, shortly after the check point there is a Pokey blocking a Yellow Warp Pipe, go down that Warp Pipe and with your Super Leaf, repeatedly bounce off Goombas until the time limit ends.
Then, as you died and still have the check point active, get more Super Leafs, and repeat. I get around 100 more lives to every 1 death, if you know a faster way. I’d love to hear it. Please note, that is 100 more lives in the space of around 200 seconds. It’s worth it.