How to develop an LTI1.3 tool in Java and integrate it into Open edX?

I am learning how to build the first “Hello World” LTI app in Java. I found a lot of information online and also downloaded some code from Github. But the code is still a little hard for me to understand. So are there some simple examples that can help me learn step by step how to build an LTI app? Or is there a tutorial that I can learn how to develop?
Anything pointing me in the right direction would be much appreciated!