I’m not sure there’s a better way than practice. There are some typing games we used in school, looking online there’s still quite a few places that have those up. You can play right in the browser and they help you get faster while setting a goal and tracking progress. If you’re not feeling the games, then maybe typing through song lyrics while the song is playing or transcribing any sort of speech might be more of a fit. 15–20 wpm is quite a bit, really. Maybe starting with a quick, free, course online would get you the speed while just learning the “right” way from the start. It’s probably the fastest way as well and is going to make any other practice more beneficial. Good luck.