The mechanics are simple, but the hard part is to try figuring out how to complete these tracks as fast as possible.
I already tried to TAS Trackmania DS, and I gave up fast, while I was trying to beat
the fastest time ever done by a human being, without any tools, on the first (yes, the first) track!
TASing is hard, and it requires a lot of patience and determination. These are two basic things you need to become a good TASer. Sadly, I hardly have these when it comes to TASing. It needs to go well, or I have chances to give up. That's why I think I'll simply be a Publisher here (unless we get to finish that Rayman GBA TAS).
If you have these two traits, no matter how hard is a project, you can do it.
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Now, for a more specific tip about your Trackmania DS TAS.
I'd suggest you start over your project, and work harder on each tracks. You need to achieve the smallest time on each track. A requirement is to beat the best unassisted record. If you submit a run without doing it, the judge will reject your submission. If you find time-saving glitches when you work on your project, exploit them. If you find any kind of shortcut on a track, use them. Do everything to achieve the smallest time possible, but don't hack the game, and don't use cheats such as Action Replay and Game Genie cheats. In fact, don't use cheats at all.
Just finish these tracks as fast as possible. Use glitches and shortcuts if you find some of these. Oh, and don't forget to make a lot of save states!
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On a side note, I played a lot of Trackmania Nations, and I do think the DS version is basically the same.