difficult to do a tutorial because everybody will have a different midi file.
the principles are that the composer must work in C major. Say you have 4 measures of a midi file accompaniment in C major all you have to do is a sequencer to composer copy of drums, bass and up to 5 other accompaniment tracks for those particular measures.
The problem is most midi files will not have 4 measures you want to use in C major. So you need to transpose the accompaniment measures. If you have 4 measures in G major, then that is easy because there ia a transpose function in the composer to adjust the number of semitones to get to C major.
However if there is a chord change in the midi file during the 4 measures you want as a pattern you must transpose each set of measures separately to achieve a constant chord progression for the sequencer to composer copy.
I use an old out of date program to accurately calculate the chords from midi files, another program Midi Connections has a module that calculates chords, not quite as accurate, and there is also a chord calculator in the latest versions of GnMidi. It is probably easier to transpose individual measures in a computer sequencer if the chord changes are complex.
Once you have C major or at least a constant chord to transpose the problem is solved.
It is perfectly possible to keep a chord progression in the composer, say a 12 bar blues as long as it starts in C major. Then all you have to do is press any starting chord and the chord progression loop will play from your starting chord without needing further left hand input until you change to another variation.
All of this is applicable to the 2 tracks of the pads too, 16 measures can be copied in both cases. The solo pads are a different case.
If this is too much work try just the drums from a midi file, requires no transposing and can sound good if substituting some live drums in technics format. Next step is bass, this would be simple to edit too. These tracks could be combined with any of the millions of combinations of accompaniment tracks in the easy composer, if you want.
Personally I found EMC very useful, and the Song to Style disk works well too if you try a couple of passes in manual mode with different settings and then pick the best variations and fills.