Leave enough headroom. About 3-6 dB on your whole mixdown should be fine.
Add a bounce of the whole mixdown as a ‚before‘ reference.
Label your stems clearly!
Make sure not to forget to include return tracks when available.
Export your stems with at least 24 Bit and 44.1 kHz without dithering applied.
For stem mastering: Make useful groups like: kicks, bass, synths, drums & percussions, other stuff
For mixing: Send only the tracks you want to be mixed. Clean up your tracks (delete unwanted clicks, noises and so on...).