Knowing what it's supposed to sound like: for this I recommend radio and TV. This will help with aural comprehension as well.
Knowing how to make the sounds: for this, learning the IPA and looking at IPA pronunciation guides is very useful.
Putting it into practice: Find a native speaker who is learning your native language. Get coffee once a week. Speak English half the time and your native language half the time. If you can't make that work, try dictating to your phone/computer in English. If your pronunciation is good, you'll get better accuracy. Gamify it for yourself if you can.
I started working on a few projects with one engineer who reads English but cannot speak it. In a matter of weeks, my Hebrew pronunciation, comprehension, and fluency went through the roof. Since then, I've been started making friends with people at stores near my house, making phone calls to solve things that could be solved with an online form (and Google Translate). I'm still nowhere near fluent, but I gained confidence from being forced to speak and write.
The main thing is, just be prepared to speak, not be understood entirely, and make mistakes. The other day, I accidentally told the clerk at a store near my house that I didn't need a bag because I had large cunts instead of large pockets. We laughed, I survived, and I'll never get those words confused again.
Of course, it won't magically make your pronunciation perfect, but it will help your "speaking without thinking" pronunciation and it's Easy to do
Get audio files of spoken English, preferably of someone with a timbre similar to yours (YouTube is a good source, download some videos and extract bits of audio with Audacity). Listen and repeat, initially pausing and repeating after each sentence, then repeating along with the audio. Pay close attention to things like connected speech patterns. After a couple of months of doing this for 30-40 minutes every day I started noticing improvements in my spoken English.
[1]5 Awesome Apps to Help Your Students Improve Their English Pronunciation (2019):
https://ellii.com/blog/5-awesome-apps-to-help-your-students-...
[2]Meet ELSA - Your personal AI-powered English speaking coach:
My French is pretty limited but native speakers have told me that my pronunciation is good. I credit it to this one simple trick. To use it for English though, I guess it helps to have encountered native English speakers trying to speak your language, so you know what their accent is like.
Other than that, I guess it could help to work with a coach with ESL training, maybe for an hour per week or whatever. I'm sure you can find someone on Craigslist like that.
Offhand, it might sound dumb and a little dated now, but a show that I think shows off a lot of interesting speech patterns to check out is Buffy the Vampire Slayer: there all kinds of English-speaking accents and very interesting colloquial style speech patterns there.
If you want to try more academically check out this app from the university of Iowa but it needs studying. https://play.google.com/store/apps/details?id=edu.uiowa.uirf...
English is the hardest to perfect, because of the numerous loan words which defy any systematic rules for correct pronunciation when you read. Then you also have the problem of accents and dialects. Even within a single country there are differences. In the USA: New York vs New Orleans, in the UK: London vs Manchester or Ireland / Scotland.
You could record some TV or radio program, repeat what you hear onto a recording and compare. Best approach would be to get a speech coach. They will explain correct mouth, tongue, etc placements.
I tried the Elsa app and found it pretty well done. It's an app with exercises tailored to your native tongue and they use the mic to give you a score. TBH I didn't use it as much as I wanted, so maybe after a while it decreases in value, but the impression I had it was very good. (I'm just not too motivated to improve my accent.)
You can still try and be competent in a year or two, but you'll sound weird saying certain words.
Read more, speak more, and try to not translate from your native language to english.