May have a look at this tutorial:
I think it's one of the best tutorial series to get into deferred rendering. And of course there is lot of space for improvements.
Maybe it's a little bit old but the idea behind still works.
tl;dr: Draw a sphere mesh in the lightpass for every light. You can use instancing here or try it first with single drawcalls.
Then use the depth information and the albedo-color from your model-drawpass to light the scenery.
Only pixel within the mesh will get checked and colored if there are inside of the lightsource.