Let’s start with the positions of the notes, once you have grasped this the rest becomes amazingly easy.
For some of you it might be strange to call the thickest string the first string, but it is the first string that your hand comes across when playing.
As you can see there is a zero fret and the nut is one position lower.
If you are a guitar- or bass-player it is best to orientate on the second thickest string at the 1° position. This is an E, just like on the guitar and the bass, in fact this is the same note as the lowest note on an normal 4-string bass-guitar.
If you count from there, you can see that you have the same notes as on a guitar where there is a fourth between the 4° and 5° string instead of a third: E, A, D, G, F, C.
You also have the same notes on the 1° dot (position 3: G, C, F, Bb,…), on the 2° dot (position 5: A, D, G, C,…), and so on.
In other words it is like a bass-guitar that goes one fret and one string lower and that goes up in fourths (no third between the 4° and 5° string!).
Because of the tuning in fourths, the position of the notes in relation to the fundamental note is ALWAYS the same, regardless of note or position you start with.
The fifth is always two frets up and one string up
The octave is always two frets up, two strings up.
A major third is always one fret down, one string up, minor third two frets down, one string up.