My suggestion is to use flashcards with spaced repetition, but don't let that be all you do. The reason, it think, that Children are so good at learning their native language is because they are always surrounded by it. Repetition is always good, and the way your are doing it isn't bad, but it is only one piece of the puzzle. Try to surround yourself as much as you can with English (or any other language you try to learn in the future). Also, speak the words as you learn them, and try to put them into simple phrases. I find that children's stories and fairy tales are a great way to learn, because they offer simple grammar and vocabulary in the context of sentences. Listen to English music, read English news articles, if that interests you. Watch English movies and Television shows. Most importantly, speak it. As with learning anything, repetition is key; but try to make the repetition as enjoyable for yourself as you can. The more fun you can make learning a language, the better you will learn it.