When I started doing stand up comedy 12 years ago I realized I needed to work on my accent[1].
Here are some tips,
Pronunciation of english sounds is influenced by your first language.
Understand phonetics.
https://www.reddit.com/r/LearnJapanese/comments/fohull/a_yea...
Follow the links to Pronunciation: Basics, prosody and phonetics on the page above and understand how sounds are made.
If you don't understand the mechanics of tongue position, lip shapes, where the sound originates (front of the mouth or back of the throat) then you may not be able make the right sound.
If you can spend money get expert help (speech therapist, though I haven't done this personally).
Practice. Speaking involves physical muscles so practice those muscles.
Record yourself and listen ( after a few days). Observe pronunciations that need to be fixed and repeat.
In time, the mechanics of tongue position, lip shapes etc., will become subconscious and you will not have to worry about this consciously and you will speak naturally.
1. Most recent stand up comedy performance. https://youtu.be/845a6wyO5h0