My teacher told to me:

3) Tune your half steps.

Ideally, you should have half steps between your fingers with a nice big space between 2 and 3 which is flexible enough to be tempered up or down. Using a tuner or adjacent open string, get your 1st finger solidly in tune. Keep your finger exactly in that spot as you place 2nd, which will need to be lower for about 80% of you. Once that’s in tune, go between 1 and 2 and really get a feel for the space between the fingers. Repeat with 2/3 and 3/4 until you can proceed from O, 1, 2, 3, and 4 without too much grief from the tuner needle or a displeased open string interval. Having reliable half steps between the fingers makes it easier to relax your hand and gives you this amazing ability to push your hand out of shape with extensions and unconventional techniques and still know where you are.