Pick a project and build it. The trick there is that it must be something you care enough about to carry through to the end. That's the hard part. You can get 80% of the way on a lot of projects easily. Or at least it seems like 80%. The other 20% can end up taking just as long and since it's often the least interesting part to you, it can be a slog to get through that to something you can deploy and share.
If that doesn't work for you, and I'm sure it won't for everyone, I would look for a chance to try and teach it (perhaps through a local school or create some videos to share online). Anything I have ever had to teach anyone forced me to learn it better than if I had learned it any other way because I had to find examples, counter-examples, and understand all of the parts of something where I might have otherwise skipped parts if I didn't think I would ever need them.
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.