My 3 cents

1. Be part of a Mendix project.
2. Ask questions to your fellow developers
3. Knowledge checks learning paths  < besides knowledge checking perfect for understanding way of phrasing and level of questions.

Aim on these learning paths:

Practical tips and tricks on Scrum, Project and team server; https://academy.mendix.com/link/paths/61/Unlocking-True-Collaboration 

XPath: https://academy.mendix.com/link/paths/62/Constrain-Your-Data-Using-XPath

Page building: https://academy.mendix.com/link/paths/56/Create-an-App-with-Advanced-Page-Building

Microflows: https://academy.mendix.com/link/paths/42/Building-an-Event-App-Using-Microflows

Domain modeling: https://academy.mendix.com/link/paths/59/Expand-Your-Domain-Modeling-Skills

Integrations, Import/export Excel, Import/Expose Rest: https://academy.mendix.com/link/paths/44/Importing-and-Exporting-Your-Data