i know this might seem like total bs, but play the game if you want to get better
a system i used to use before i retired (or did i quit? idk what the right term is lol) was what i like to call "the star improvement strat". basically you pay attention to how many stars there are underneath the song's title. keep playing maps that have the same star count and keep improving until you think you're ready for another star. if you're really confident, you could even jump to a whole new difficulty
i personally think that if you manage to SS at least 3 maps with the star count you're working on, that's clear sign that you're ready to add on one more star.
when i talk about stars, im referring to those little stars underneath the songs title.
(not my screenshot btw)
hope this made sense for you. good luck and i hope you enjoy osu!