I found the best starting point is to settle on a DAW, then do a tutorial on that DAW and gain a good foundation of how it is used.
Then you can go pretty much anywhere for mixing type courses, of course ones that use your DAW of choice are a lot more straight forward but not really necessary.
The thing is though if you are only mixing in bass or bass/guitar to a backing track most courses will be way more than you need.
If the above is what you are looking for, then I would suggest starting here (its free!).
Cover’s multiple DAWs and multiple ways of adding bass to a backing track and also video. The instructors are amazing 
and the content is focused on recording yourself and bass. It all applies to guitar too.
If you are looking to go further I suggest picking a DAW and learning it first, then go deeper.
I took an excellent course on Abelton on Udemy that taught me a lot about music production in general as well.