Preparing files for mixing
When preparing your files for mixing, make sure to convert all midi data to audio. Bounce each track seperately as a continuous audio WAV file with all tracks starting at bar 1, beat 1, 0 ticks). That way, when the file is imported, all tracks will line up as they should.
All of the tracks should have all of the mix plugins taken out, including compressors, EQ, delays, and reverbs. The files should completely unprocessed. If there are specific elements you want, bounce the file twice. One file processed and the other unprocessed. The files should named correctly and the tempo needs to be sent. You can have the tempo in the name of the files (eg. snare_bpm 104.wav). Also, make sure the files are not clipping. The volume should not be too high.