It’s pretty simple: get as many 5s as possible in APs, particularly in your subject and related classes (ie science/math for STEM, social science/English/foreign language for arts). You’ll have to show all scores, so don’t take the SAT/ACT multiple times for “practice”. One and done is best (so taking in sophomore year isn’t a great idea, if possible use the unofficial PSAT to see where you stand). And don’t do AP exams if you aren’t going to get 4s and 5s.
And narrow in on your academic focus: you just study that subject, so ECs (in as much as they are relevant, which is much less than the US) should be related to the subject. Here’s some good advice, including the classic line that they are looking for excellent students, “not second-rate historians who happen to play the flute”: