This part of the Documentation does not include how to use Global Illumination! Please navigate to the "Global Illumination" Page on the Navigation.
First you need to simply download the Asset Pack into your Project and you should be ready to go. I recommend that you take a look at the Playground Level inside the Demo folder first.
Creating a Light is as simple as to Drag and Drop one of the Light Painter Actors into your scene. You should already see the Indicator Billboards of the Light Painters. They will tell you what Type of Light you are using and what Color it currently is.
The Lights are not visible inside the mesh by default. This is expected behaviour and a Performance Optimization actually! The Lights are 1-Sided Materials put on Meshes, which makes it impossible to see from inside.
The Simple Light Painter swaps out the Materials and Meshes of the Light Painter during Runtime AND also in the Editor. This allows the Lights to be Culled by the Engine and save Draw Calls. To activate this System you need to do 2 Things:
Run the Editor Utility Widget of the SLP, this will allow you to see the changes in the editor! (This requires the "Editor Scripting Utilites" Plugin to be enabled)
As long as this Widget is opened, all the Light Painters you are overlapping in the Editor will swap out Meshes and Materials to be Visible from Inside.
If you have done both in your Project, you can continue with the next pages of the Documentation!
Press T in the Editor to disable/enable selection on transparent objects. This is useful if you do not want to click on the Light Painter Meshes all the time.