In case you don't want to watch the video, I took some notes. I recommend watching the video anyway since Krashen is an entertaining guy.
The brain is bad at conscious learning, but good at language acquisition (Dr. Krashen says this is "understanding" language completely involuntary). Language acquisition requires: comprehensible input, interesting (downright compelling) content to consume. Dr. Krashen says the ability to SPEAK is the result of the comprehension of the language. Writing and speaking by themselves do not have large impacts. Minor error correction does not have large impacts.
Conduit hypothesis - Universal stages of language acquisition. Stage 1: Stories (fiction). Stage 2: self-selected reading for pleasure with increasing difficulty
Start with stories, they give us everything and lead to READING. Hop if there's anything you don't like. Begin with a somewhat long period of EXTREMELY EASY STORIES (use US foreign service tiers to determine how long you should do it, maybe up to 3 years…). Graded readers are excellent for language acquisition. Forget tests/quizzes/questions/interaction/volcab memorization… Just read/tell another story, that probably leads to better results (volcab, test scores, etc).
Reading is the most important thing. As long as you pick what you read it'll be good. But reading fiction is more effective (for whatever reason). Spaced repetition/exposure will keep up your knowledge of that language even if you never use it in daily life.
Writing, on its own, is extremely derivative of what you READ. REVISING what you've written is the way to get smart.