you will need to have portal 2 authoring tools beta for this to work.
if you have it installed continue reading.
hope you enjoy:)

go into portal 2 authoring tools and start up the hammer world editor.



















then load a file from C:\Program Files (x86)\Steam\steamapps\common\Portal 2\sdk_content\maps\instances. make sure it is something like wheatly, a light, faithplate etc.

once you have hammer pulled up and something loaded you will want to add a light to the item it can be any kind of light now position the light in the place where the thing you loaded lights up. if there are multiple instances of the object do those to.

hope you enjoyed my guide.
have a great day :)