Play easy maps to practice accuracy. Try to aim for SS scores on 1-2 star maps and at least above 995k score. Do this a lot to get some good practice. HardRock mod is pretty useful for acc training as well, although I personally prefer going on Etterna and using its various Judgement systems to train accuracy, since you can adjust it to whichever one you're most comfortable with. For normal gameplay, I use J5, but for acc training I use J7 and J8.
It's also good to practice your consistency with accuracy, like trying to SS easy 3-4 minute 1-2 stars. Remember, like all things, this takes time. Be patient, with enough practice, you'll be getting 1M scores on 3 stars.