More of a complex one that will require lots of trial and error. Screens must be made for all entities that need them. Examples are already present and tutorials have even been made from research.

See these to help:

• https://forum.islandearth.net/d/10-forge-modding-tutorial-1-14-creating-custom-guis-1-3
• net.islandearth.taleofkingdoms.client.gui
• net.islandearth.taleofkingdoms.common.entity

Going to close this as this is mostly done for the guild now!