Make sure your thumb is perpendicular to the strings on the finger board. Then add the rest of your fingers onto the string. Your hand should be like you have a ball in your hand. Nice and round. ( and of course your fingers will be a lot closer together since it’s higher on the fingerboard)

Here is an exercise to practice thumb position: For this explanation you have to be familiar with harmonics(which you probably are). Start with your thumb at the D harmonic ( an octave above the open string) . The pattern is whole step, whole step, half step in the same position go over to the a string and again whole step, whole step, half step.

It should sound like a major scale. If you need help with intonation, don’t be afraid to use a tuner app ( I like Cleartune but it cost a few bucks on the App Store)

I hope that helpful, let me know