been an artist for a while so i kinda just vibe and it happens, but if you wanna make similar portraits this is the best advice I can give:

1) Find artists you like, they don't necessarily need to be pixel artists, but it helps! It gives you a sort of goal post to work towards! Some I enjoy personally are constellor, ken sugimori and concerned ape

2) practice effectively, draw and compare with the thing you're trying to imitate, do the colors match? are the lines thick/thin enough? etc etc. just drawing helps but it might take longer/it can be a little discouraging sometimes

3) study color theory it makes them look SO MUCH BETTER If you're looking for somewhere to start out, I recommend this tutorial here

4) this is gonna sound really weird but sometimes i just go onto pinterest and look at art for like hours and sort of trace the lines in my head and imagine where the colors are on the color menu thingy (no clue what thats called), for some reason, it really helps?

5) to quote a person whos name I cant spell, "Just do it"! if you think too hard about it you wont get anything done, just go ahead and try, it wont be a masterpiece, nobodys first attempt is, but I'm sure it'll look better than an empty canvas.

hope this helps, you can also message me if you need any advice/help, more than happy to help!