We have created a video detailing how to use the Asset Editor to add custom items, tools, and blocks to Hytale. 

You can check it out below:

https://www.youtube.com/watch?v=9CDVaFO3Jrw

ZIP file for reference:
https://drive.google.com/file/d/1-tT6CX697HXcaMXOsZ_5-sGcrz-Q70-T/view?usp=sharing