as someone who just spent the last month stressing over letters of rec i would say:

-target professors who know you, preferably from smaller sized classes. if you have no choice then you can ask a professor from a big class you did well in, but you'll probably get a more generic letter.

-send them an email talking about how the class positively impacted you, then talk about your future and finally bring up the letter. definitely offer to meet in person to discuss your goals and strengths. provide a resume/coursework/personal statement - helps them write and gets you better results.

-ask 2 months ahead of time and be prepared to send follow up emails. professors are busy and they probably aren't gonna knock your letter out right away. one of my recommenders didn't even get around to responding for 3 weeks. if they agree to write you one, don't be afraid to keep them updated on deadlines, because they might not remember.

-lastly...don't stress it too much. this is part of a professor's job, you'll get what you need. just make sure to act in a timely manner yourself!