Make sure you save your tracks starting from the beginning of the project. In most DAWs (Digital Audio Workstations) this can be accomplished by setting a loop for the duration of the project.
Please name your tracks according to what they are (i.e. if its a kick drum, the track should be named 'KICK')
Make sure you have any plugins on the mix bus (also known as the master bus) disabled. These can interfere with the mixing process. You can leave all your processing and effects enabled on your tracks. If I need any dry stems from you, I'll let you know.
Your DAW should have a feature for automatically batch bouncing all your tracks at once (it's frequently in the 'export' menu). Create a new folder with the title of the song, and bounce that soloed track into the folder you created.
When exporting all the tracks, the files should be saved as .WAVs, and the settings should be set to a samplerate of 44.1kHz (or higher, if that's what you're using), and a bitrate of 24. Any normalization features should be disabled.
Run a Test
Create a new project, and import the new stems you created to make sure all the files are in proper working order. If all is well, they are ready to be sent!
Send it Over!
Compress your folder of the tracks into a .zip file, then send them to using either wetransfer.com, or whichever transfer service you prefer.