There are a lot of different ways to tune a double bass.
I’ve tried just about everyone over the years, and I’ve settled on two methods that I use pretty much every day.
Open strings with a tuner
I start every day the same way on the double bass, which is tuning each open string.
I either use an app like Tonal Energy or a clip-on chromatic tuner.
It’s so much more convenient to use the clip-on tuner in an ensemble setting. I’ve tried to use my phone for years, and that works fine, but the problem is that if it’s loud–which it often is–the phone won’t really pick up the bass. Then I put it on my bridge and the phone’s kind of precariously perched.
So this chromatic tuner works great. I just play each open string and I wait to make sure that I’m right on the money.
Using the “tuning harmonics”
The other way I tune is kind of a traditional way on bass, and it’s using the harmonics over what would be 5th and 7th fret on the electric bass.
The reason why we bass players like to use those, especially in an ensemble setting, is because again, ensembles are noisy.
The reason why we use these harmonics instead of open strings is because the pitches of the open strings are so low that they make discerning the intonation challenging, especially in a noisy environment.
I’ve been playing the bass for, I don’t know, a long time at this point(!), and I still can barely tell if those open strings are in tune without using a tuner or without using these tuning harmonics.