If this is your goal, the best way in my experience is to use it as much as you can, minimising the translation element as much as possible.
Composition books will give you English sentences and ask you to translate them, the idea being that you focus on how to express certain structures or concepts in Greek (conditionals, narrative or whatever it might be). This is of course extremely useful, but it will never really be “active” inasmuch as you aren’t expressing your own ideas, and the fluency you acquire will always be limited because you are first thinking of the English word and then translating it - e.g. “Table. What’s that in Greek? τράπεζα”. It would be much better from a fluency perspective to just look at the object and think of the Greek word (that’s one of the reasons why language teachers use flashcards with images instead of asking students to translate).
So my advice would be (as a supplementary activity - you’ll need to keep following a coursebook of some kind to get the language input) to look at images and describe (first orally and then in writing) what’s happening in Greek, only using English when unavoidable. Obviously, you won’t know the words for everything in the image, so you’ll have to find out. If you had a teacher, this would be really easy, as you could just point and ask, do a mime, describe it using other Greek words etc… But as you don’t, you’ll have to look them up in an English-Greek dictionary, such as this one:
https://artflsrv03.uchicago.edu/efts/woodhouse/woodhouse_search.html
Only use the dictionary as a last resort - if you can’t remember a word immediately, give yourself time to think about it, and if you’re unsure if you’ve remembered it correctly, make the sentence with your best guess and then check in the dictionary afterwards.
This can de done at any level - a relative beginner might just describe the people and objects (present tense only) whereas if you’re more advanced you could speculate about how the people are feeling or construct a whole narrative (e.g. what happened before and afterwards, using a variety of tenses, aspects and moods) or a dialogue.
The disadvantage of this is that there’s no answer key, and you’re going to make loads of mistakes, but that’s normal. Ideally you would have someone check them for you, but otherwise just go back periodically and look for errors yourself.
If you want to be really authentic you could use images from Greek vases.