Go slow and listen
1. Do not watch a tuner while you play. Use the tuner to tune before every practice session. The benefit of a fretless instrument is better harmonies that are not possible with a fretted instrument. Think back to your orchestra days when the conductor had everyone make micro adjustments to hit that perfect harmony.
2. Do not look at the fingerboard while you play.
3. Do not lose contact with the string (left hand) when you shift. Move slowly and listen as you slide into the note. Check against open strings. That can be octaves, major thirds, minor thirds, fourths, fifths, etc. Eventually, you won't even think about.
I also play cello and double bass. This is standard intonation practice.
1. Do not watch a tuner while you play. Use the tuner to tune before every practice session. The benefit of a fretless instrument is better harmonies that are not possible with a fretted instrument. Think back to your orchestra days when the conductor had everyone make micro adjustments to hit that perfect harmony.
2. Do not look at the fingerboard while you play.
3. Do not lose contact with the string (left hand) when you shift. Move slowly and listen as you slide into the note. Check against open strings. That can be octaves, major thirds, minor thirds, fourths, fifths, etc. Eventually, you won't even think about.
I also play cello and double bass. This is standard intonation practice.