Flashcards work for a while then stop. Why?
Isolated words decay because nothing retrieves them. Cards with a sentence you actually encountered survive, because the context provides the hook. The rule of thumb is to add nothing you have not met in the wild.
How do I explain recursion without factorials?
Use a task that is obviously self-similar in the physical world: looking for a key in nested boxes. Open a box; if it holds a key you are done, if it holds boxes you repeat the same instruction on each. The base case becomes obvious because an empty box is obvious.
How should I structure week one of learning a language?
Sound before grammar. Spend the first week on pronunciation and the hundred most frequent words, because everything later is easier once your ear can segment speech. Grammar drills in week one produce people who can conjugate and cannot hear.
Does this still hold if I am working alone?
It gets easier, not harder. A fair share of the difficulty is coordination, and that disappears. What remains is remembering your own decisions, which is what writing them down is for.