I can only speak from my limited experience. The best way is the one you find for yourself. You might never find it and even reach the top doing something that could be considered sub-optimal, but remember the most important lesson of them all: unless you're learning objectively wrong stuff, there's no terrible way to go about it; just keep up the pace and don't ever stop. You can find ways that suit you better later, just finish today's work first. You should of course try flash cards, i.e. Anki, to see if you like it. The many premade decks will help you get started on most subjects you want to tackle. Later, you can even make your own decks. If you end up hating it, minimize its usage or stop completely so that you don't start hating studying. I admit I'm a bit ignorant on the alternatives; Anki just works so well for me I didn't spend much time using alternatives.

There's only one place to start and that's being able to read (and ideally write) kana. It should take you a good day's worth of work to get there. When I started out, there was this website that showed you a character and you'd have to type in its romanized equivalent. This really lets you grind recognition quickly. I suppose you could also try Remembering the Kana book, or find something that's more to your taste.

Getting a 30 second lesson on how to grammar is your next step. It will allow you to start reading some very simple manga (like Youtsubato) that has furigana (so no kanji knowledge required). Perhaps there are better quick start guides these days, but Tae Kim's guide is the one I used and the only one I know. It's bare bones and I think it even has a few inaccuracies, but it gets the ball rolling so that you can start reading and listening ASAP. You want to read at a decent speed and you shouldn't let your kana (and later, kanji) skills rust.