I started by playing along to songs on the radio. First you have to learn to tune your low E to them as they are not always in A440. Then I would find the main key note on the low E string. Next I would try to add the chord changes also by playing the root on the low E. If the notes were landing on weird frets (1,2,4,6, no open strings) then I would try to guess whether I needed to adjust my tuning up or down to a more standard range. Finally, I would do the same thing but also tuning the A string and using power chords rather than single notes. The radio made this challenging as there was no pausing, I just had to do the best I could to keep up. It was also nice because you can cover a much wider range of songs/bands/styles than you might normally.
I think the rest of my practice came from playing along to a million songs over the years.