You know what swing is, right? It's when the offbeats of a song are slightly delayed, like in this song: https://www.youtube.com/watch?v=5Up5wDdL-Gs

How would I do this in Super Mario World?

Oh, I see. I was hoping there to be some kind of automatic way, but I guess this will have to do.

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I don't wanna be a Flopping Cheep-Cheep, so I became something else instead. Wait, I don't want to be a Terrapin either. -_- Whatever, I'll just stick with a Flopping Cheep-Cheep.

This is an advanced tip. Just throwing this out there if anyone cares: you can use the = command to specify a length in ticks. So, c=48 is equivalent to c4. c=24 is equivalent to c8 etc. This is helpful if you want to fine tune the amount of swing. For example instead of c8 c8, or c=24 c=24, you could do c=25 c=23, or c=26 c=22 or whatever. Best way I've found to tweak that is to get a basic drum groove going for the song, and experiment with swing on the channel that needs it most. Obviously you want to get this right early, otherwise you may have to redo a lot of things which is annoying. And you always run the risk of going a tick or two off sync with this method if you're not careful. But when triplets are too swung, or you need some sort of unique swung feel, that's sadly the only intuitive way to do it.

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Make more of less, that way you won't make less of more!

It would be unwise to write swing rhythm with timings like c12 and c24. It should work, but in my experience, it doesn't. I ported Playing The Game from Mario Party 1 which uses that rhythm, but MIDI to MML programs won't port using those { } so the rhythm of the port got all screwed up. I had to manually add in { } with the correct note lengths. After reworking those sounds, I found that the swing rhythm goes in this pattern:

{c4 c8}{c4 c8}{c4 c8}{c4 c8}

That's one measure of notes.
As opposed to standard 4/4 time rhythm:

c4 c4 c4 c4

If you port a song with swing, it may look like this:

c6 c12 c6 c12 c6 c12 c6 c12

If you find this to be the case, run a Find and Replace and replace all the 6s with 4s and all the 12s with 8s, then be sure to insert curly brackets around them and delete the extra .s that appear due to porting errors.