First off, keep this in mind...
Most DMs aren't voice actors
And most players are fine with that. Most of us aren't Matt Mercer or his players.
Most DMs aren't experts or even accomplished at accents and voices and everything else. Most players are perfectly capable of maintaining immersion while their DM just talks in their own natural voice, perhaps adding descriptive flare to describe how they are talking, rather than trying to mimic it.
But, if you want to branch into this direction....
You're going to look like a crazy person if you do this in public, but the only way to 'git gud' at accents is to practice them. Go on Youtube and search for people speaking English with different accents. Pay attention to how they pronounce different sounds. You can sometimes even find guides that will walk you through the nuances of a particular accent.
And once you have that, practice. Walk around and talk to yourself (or someone you're very comfortable with) in a funny accent and keep messing with it until it sounds a way you like.
Practice Altered Diction
Part of a character's voice is the specific way they talk. Listen to other people talk. Notice how they hold different letters for different lengths of time. Notice how they pause in different places than you do. Note how sharply they say consonants, or if they mispronounce common sounds. Notice how quickly or slowly they talk, how much they vary the pitch of their voice as they do so. Do they actually finish saying all their words down to the last consonant, or do they just let them trail off?
Then, as before, practice mimicking it.
Practice Different Speech Patterns
Again, listen to people. Listen to their verbal tics and the ways they say things. Do they use complicated words? Do they have a tendency to preface sentences with a particular word or sound? Pick important NPCs and figure out how they talk.
Does this person frequently use contractions, or do they say the full words? Do they lop off parts of words, like saying 'em instead of them or 'ey instead of hey, or perhaps they would say the word perhaps as "per'aps."
Practice messing with the shape of your mouth
The expression you are making with your mouth when you talk can warp the sound of your voice. A voice spoken through a sneer actually sounds different than a voice spoken through a neutral mouth...which sounds different from a voice spoken through a smile. Mess with the shape you hold your mouth in while you talk and listen to what that does to your voice.
Curl a lip up, smile, frown, narrow your mouth, smile with only half your mouth, talk through gritted teeth, try talking without putting your teeth together at all.
Once you know what these facial positions do to your voice, you can use them when you want to. A lot of people can actually hear the expression you're making when you talk, and that can help convey mood and temperament.
There's only so much you can do about the way your voice sounds. It is very difficult to sound dramatically different, but you can create convincing voices despite that.
Play more with different ways of talking, rather than just trying to pitch your voice around. I have characters in games I run that my players can identify by voice, even though I'm using my natural pitch and (lack of) accent--simply because the character has a distinct diction.
So, in your case, perhaps Strahd doesn't have a big booming scary voice. He is corruptive, seductive in his own creepy way. Perhaps he has a voice like silk that slightly draws out his softer consonants. Perhaps, given how old he is, he speaks with an older style dialect, using older terms and forms of address. He is a nobleman, to boot, so he likely speaks in a refined manner...but with just a touch of a sneer. And, perhaps in reference to his undeath, perhaps his voice varies in pitch only slightly while he talks.
Speaking plainly, I'm a guy. I can't mimic a girl's voice very well. I mostly just end up sounding like a guy talking in a higher-pitched voice. But I definitely have voices I use that are sufficiently 'feminine' that my players can recognize them as 'female' by sound--and are consistent enough that my players can identify the specific character they belong to.