Conversational to Mastery

From the start, speak at least an hour a day in the language. Have varied conversations.
Ensure that your conversation is improving—not just “general language skills” through some vague list of words.
Lots of practice and study to improve those spoken sessions tends to get you to lower intermediate level.
This is the point to return to academic material and grammar books, to tidy up what you have.
To get into the mastery, start reading newspapers, technical blog posts, or other articles that won’t exactly be “light reading.”