Custom Billboard/Surface Gui like TTT, any explanation? MouseEvents?

Hi I'm TheLorizz and I'm here to ask help on creating a custom Billboard/Surface Gui, like Trouble in Terrorist Town's one (since I'm making a TTT game for roblox). This is the first time that I use a similiar thing, how can I see in real-time what I am doing with the surface gui like the normal screen gui? Also, does the surface gui works like normal screen gui? I mean, does it needs Frames/Buttons etc.? This gui needs to appear on your current mouse position, I tried something like this for this MouseEvent:

local vec1 = Vector3.new(...) -- starting pos
local vec2 = Vector3.new(...) -- mouse looking at
local par1 = Ray.new(vec1, (vec2- vec1).unit) -- ray

And I don't know how to continue, do I need to use some events like "MouseEnter" for accessing the ray?

Thanks for helping! I AM NOT REQUESTING ANY SCRIPT, I NEED ONLY EXPLANATIONS!