Preparing your songs for a mix engineer can vary. As it pertains to the preference of the mix engineer working on your song(s). Some engineers may prefer you to send them your songs in the program session they were created in. While some may want you to send them individual stem files. But before doing anything there is much preparation on your end that needs to be done.
1. Labeling- Whether you're sending your project sessions or audio stem files you need to be organized. I have yet to see an artist record vocals without having any multiple takes and edits. With that being said make sure your labeling everything from intro chorus, verse one, bridge etc. Just because you might know where and what everything is doesn't mean the person who will be working on your tracks will. This saves you time and money as the mix engineer may feel like it's not part of their job to organize your work.
2. Clean up your tracks-The dead space that often appears after your vocal recordings should be trimmed down. This includes unwanted noises that may have been caused by improper edits such as clicks and pops and any other unwanted artifacts. The use of cross-fading at edit points will help fix most of these issues. Another good technique that can be used as a counter measure. Would be to solo your tracks to hear if you can identify edits while looking away from your computer monitor. These are preventive measures against unwanted artifacts that could affect the quality of your mixes later on during the mastering phase. Where the use of heavy compression will be applied to your song, amplifying any and everything that you may not have been able to hear before the mixing stage.
3. Bypass All effects Before you go and hit the export button make sure you have bypassed any EQ, reverb, delay and compression plug-ins that you have processing on any of the tracks. As this will cause issues for the person working on your tracks. Once you've exported your tracks. There will be no way for the mix engineer to bypass your effects, once you've exported your tracks. Resulting in a delay of your song being mixed. Because he or she needed to contact you about having your songs resent with unprocessed audio.
4. Folder And Content info-Besides labeling your files you also need to label the export folder you've created. You'll want to label your folder with the song title and tempo. Also including a notepad file within the content folder stating any useful information such as sample rate and bit depth of the exported track. Once you've got everything done. You'll need to send your files to your engineer using a file sharing system. To consolidate your file size a free to use program called “winrar” or “zip file” can be used for this. Now you can opt out and send just your folder as most file sharing sites will allow you to upload just your folder. Whether its dropbox or mediafire you'll be able to upload with ease.
These are good practices for any project you're sending to your mix engineer or even collaborating with a producer/artist. Most professionals will be happy and willing to guide you throughout the entire process. But having some knowledge of how to send your sessions helps increase the turnaround time for your project. There are a ton of tutorials that can help familiarize yourself with the process, using the DAW of your choice.