I would recommend the book "Remembering the Kana" from James W. Heisig Part 1 and 2. He teaches how to remember Kana with association techniques. Although there are only a few and you could still learn them by brute force it would be easier and way faster with the book. But if you are learning Kanji his Books "Remembering Kanji Volume 1+3" are really indispensable. Volume 2 is only about pronunciation which you could also learn by using so it is not really needed.

Just learn 5 signs per day. Its simply and good way to learn jap. alphabet (ofc for me) :) I think fast learning 15signs per day isnt good idea - its so boring and you will fast forget it