It is extremely difficult to improve pronunciation by yourself. Teaching pronunciation is a subspeciality within teaching English. Relatively few teachers having training in phonetics and phonology.
A qualified teacher will make a large difference in a few lessons. You will practice "hearing" the sounds and "making" the sounds. I have taught North American pronunciation to Arabic speakers. Usually, they have great difficulty hearing the differences between these twelve words: beet, bit, bait, bet, bat, bot, bout, bite, but, boat, book, and boot.
Once a student has learned to "hear" (distinguish) the sounds (phonemes) of the language, they progress reasonably well in learning to produce these sounds. After that, lots of practice is needed (listening practice with audio books and similar resources and speaking practice with teachers or conversation partners).
Good luck !!