This may sound obvious, but I think one of the best ways to learn a programming language is to solve problems with it. This works well when they are problems that you know you could solve in a language you are more familiar with. That way, you spend most of your energy on the translation and less on the problem solving.
To that end, I've decided to work through the 2019 Advent of Code using Haskell, explaining what I'm doing as I go. Let's see how far I manage to get before other things draw me away, or I feel like it's no longer productive.
Most of these problems I've already solved in Python, C (urgh), or C++, so I'll more or less be trying to translate my solutions and referring to them a bit. Let's go!