If a topic has a link to Docs, you will need to create and document the learning path as you go. Take a look at the Geo Checklist for a complete example. Replace the link to the Docs with a link to the checklist and put a (WIP) after your link to let people know that they won't get the expert badge by finishing the list in its current state.
Try to pick what the team needs most, thinking back to recent tickets you were not able to answer is usually a good starting point. Inform your manager which one you will be working on, so that they can let you know if there is a different area where the team really needs expertise. Always try to answer tickets on other advanced topics, but when it is time to do some dedicated learning, focus on one area at a time.
GitLab API (WIP)