There are lots of awesome (and free!) online resources for app designers. You just need to know where to look!
Being an app and web development company we are pretty much obsessed with design. We need even the most junior team members to be design-savvy as they might be involved in the process in one way or another, but at the same time, we also want to be resourceful and utilize as many free opportunities for the learning process as possible. If you are still learning how to design apps and would like to avoid spending thousands on courses for app designers, you might also find these 3 ideas very helpful:
Free online courses: To junior team members we recommend that they study courses on Envato Tuts+, which boasts thousands of free tutorials, articles, and video courses. There are tips for beginners on Mobile UI design, typography, design of specific app categories, and lots more cool and inspiring information. Other course collections to check out are Lynda, Treehouse, Codeacademy.
