1. how do you open it from a player

2. how do you make one??

I would start with a video like this one https://www.youtube.com/watch?v=x08i64ftpDY

 

11 hours ago, FOGC123 said:

1. how do you open it from a player

use NetworkHooks#openGui

11 hours ago, FOGC123 said:

2. how do you make one??

look how minecraft create container,
but basically you need the container and a screen

7 hours ago, shutdoor said:

I would start with a video like this one https://www.youtube.com/watch?v=x08i64ftpDY

tutorials are not the best solution to learn something,
read the forge docs and look here in the forum, then try to create a container,
if you have problems creating it you can still ask