The challenge is meant to improve all areas of learning a language, like grammar, reading, writing, and listening. Some ideas I've been using myself and others I found from other people.
Now to start the challenge.
Focusing on setting goals:
Create a limit of how many words you want to learn in a day/week/month/these 3 months
Log how much you learned each day/week/month
Create a list of all the words so you can quiz yourself at the end of each day/week/month/these 3 months
Try to read something new each day
It can be English on Facebook, in the news, posts on SNS
Do you understand all the grammar? If not, write it down.
Try to make a log of how much grammar you know and want to know.
Use the grammar you know in future sentences/journals/while speaking.
Try to listen to an entire show/movie/song without subtitles.
How much did you know? After that put on the subtitles and figure out what you missed.
Set goals for how long/much you want to listen in a day/week/month/these 3 months.
Set a goal to watch/listen to a certain amount of shows/movies/songs.
How well do you speak?
Try to speak for at least 1 hour in your target language.
Can you already do that?
Then increase the hour. Maybe 2 maybe 5. Maybe a whole day.
You can make a video of yourself speaking about a topic. At the end of the three months you can watch it again and judge yourself.