I use Pro Tools HDX to mix. If you are using Pro Tools, in any version or format, you can simply send me your session and I will be able to load it without further changes.
If you are using another DAW, such as Logic, Cubase, etc. then you will need to bounce out your individual tracks as WAV or AIFF files. I prefer 24 bit WAV, but if you are already using another format, that’s fine. Consult the user documentation for your DAW for explicit instructions for bouncing tracks.
When you bounce your tracks, please bring the faders to unity gain (0). If your faders are very low, the resulting bounced files will also have a very low volume. Also, please remove any effects such as reverbs or delays. The only exception to this is if it is an effect that is an integral part of the sound (for example a sweeping filter on a synth). When in question, just bounce two copies of the sound, one with the effect and one without.
Most importantly, make sure each audio file is bounced from the beginning of the session, even if that means there is blank space. This way, everything will line up correctly when I import the files into Pro Tools.
If you bounce your WAV files from Logic, Cubase, or any other DAW, you might want to convert stereo files to mono whenever they contain mono sounds. I recommend using StereoMonoizer before sending the files. This will save time and bandwidth in the transfer, and make sure all the files are in the correct format.