Double-click the desktop icon to launch your preferred digital audio workstation.
Click "File," "Open" and select the relevant session from the drop-down menu.
Hit "Play" and then click the "S" icon on the first channel. This solos (isolates) that particular channel so everything else is muted, enabling you to hear just the audio. If the track is instrumental, add "Export" to the end of the file name. Repeat this process for each audio channel so you can quickly reference the audio you require.
Click on the first audio track for export so it is highlighted. Highlighting the track assigns subsequent commands to that particular audio file specifically, rather than the entire audio mix.
Click "Send To" and select "Bus 1." In audio, a bus is a mix channel to which you route multiple audio files. Routing multiple audio to a bus channel enables you to treat the collective audio files as a single entity. This is called submixing. Repeat this process for each selected audio file, so they are all routed to the same bus channel.
Highlight the bus channel.
Export the bus channel audio. The method for doing this varies slightly according to which digital audio workstation you use, but typically you open the "File" menu, select "Export" and then, when prompted, select "MP3" as the file type. This renders the selected instrumental audio tracks as a single audio file.
Name the MP3 "Song Name Instrumental" so you can distinguish it from other files gleaned from the same recording session.