If you need motivation to improve your UI design skills, consider trying the Daily UI challenge. When you sign up, every day for the next 100 days, you will receive a challenge to complete. Challenges include creating an interface for a sign up page, work out trackers, icon sets and more.
I think it’s a decent way to explore creating user interfaces because it is up to you to decide how you want to approach the challenge and how complex you want to make your UI. If you are a beginner, you can simply focus on the essential components of the UI just to learn how to design them or if you are experienced, you can try to push the boundaries and experiment with UI design concepts.
In the end, if you successfully complete all 100 challenges, you will have gained experience with designing UIs and possibly some things to add to your portfolio. You can start the challenge at any time.