How to create a Devkit

Hello

How is the way to create a Devkit for people that want to mod?

The Game is already on Steam but not released. We just decided to give out some Devkits to people that are interested to doing maps for example.

Is that an add on what we have to upload to UE 4 or to Steam or both ? Atm no clue how to do that. Some help would be super awesome.