How to prepare?

把需要考的HSK等级，所学内容全部复习一遍，然后做相应的试题，等到考试的时候会得心应手。

Try to do as many sample tests as possible. It will help you to manage the time. And work on vocabulary, write the words, use them in a sentences. Try the Chinese in flow app to review the words. It helped me a lot.

There are a number of methods you can use to tackle such a big project, with a mix probably being the best option.
If fluency is your end goal, I recommend methods such as https://refold.la/ or https://www.lingq.com/en/ involving a large amount of immersion input.
If you need to pass a certain HSK test for a job or other credential, focusing on a well rated coursebook on HSK may be better suited.
-Cchatty has HSK courses for download
-You can download premade HSK Anki decks to your phone to practice daily (do not add too many cards each day).
-Doing practice tests (can be found for free online) will be invaluable to guage readiness.

Whatever method you choose, make sure you enjoy it! You won't stay motivated easily if you don't.
加油