[S]Currently looking for an existing thread, but until then, I'll post my own response.[/S]
So long as you have obtained a copy of a MIDI file, you can run it through 3MLE (technically it's called 3ML Editor 2), which you can download at this link:
http://3ml.jp/download.html. You can easily run the program after extracting the files.
Located under "File" on the menu bar is the option to "Import Standard MIDI File..." It'll spit out the converted MML shortly after you select the file in the browsing window. All you need afterwards is rudimentary knowledge of how MML works if you need to cut it down to fit a certain composition rank.
There's also an explanation of this with screenshots, found in this full-fledged music guide:
http://mabination.com/threads/2810-Kaxiyu-s-Music-Guide.
If you need any more help, let me know, as I make my own MML/modify existing ones all the time. XD