I think the easiest thing is just to get a dialect coach. I know that's how celebs do it.
If you wanna try to learn an accent all by yourself I think you have to just immerse yourself in it and then practice a lot. What I'd recommend is to watch a series where everyone has that accent, then try and pause to repeat what they just said. I learned a bunch of different accents growing up from mocking teachers I hated.
Hope this helps!
This is a great answer. I don't think I have the bandwidth for a coach right now, but honestly watching tv shows and practicing that way seems so obvious now.
Go ahead and try it! Which accent are you trying to learn, btw?
Italian mobster accent HAHA