Using this site
The key way to develop a new language is to use it a lot, over a significant period of time. To do this, you need a huge amount of authentic materials which you can manage easily, and which you'll enjoy being involved with.
For a given sample of INPUT language (reading, listening and watching), the difficulty will generally go from left to right. To develop OUTPUT language (speech), go here.
Reading
This is the key to getting exposure to large amounts of language.
You read much faster than speaking or listening, and the text means that you can check back whenever you want.
The key things are to have at least 96% word accuracy, and to understand and enjoy what you are reading.
Reading + Listening
Listen to some text being spoken, at the same time as reading it.
This helps you to understand and learn pronunciation, and the ways in which words are spelled
Videos + Subtitles
Subtitles can be used as additional cues to generate meaning, and to link in with listened language.
Videos
Meaningful and interesting videos give additional cues for meaning and social contexts.