Please deliver your mixes as either multiple-mono or stereo-interleaved WAV files, in the highest native resolution possible. Please print mixes at their native resolution and bit-rate (the same ones they were mixed at).
Do not worry about loudness or making your mix loud (we’ll do that).
Do not overly compress or limit your master fader / stems (unless it’s a creative decision).
Do not perform any SRC (sample rate conversion) on your mixes (again, we’ve got this).
Do not add any dithering to your mixes.
Do be sure to leave enough headroom for us to work with (-6dB for OCCASIONAL peaks is a starting point).
Do make sure your mixes are truly “final” before they are sent to be mastered.
Short version: Just make your music sound as good as possible, fix as many issues as you are able to, and let us take care of getting the levels competitive and fixing the rest!