For SP players: 
You just need to download RDE mod and extract the lasersight.asi plugin and paste it to the scripts folder


FiveM users…. well
Just like above but you need to make it compatible with the latest fivem game build…
To do that, you need a tool called Asi Version Injector created by ZeroDream-CN (github). 
Then by looking at the fivem docs you need to type in the number of the newest gamebuild. 

It's an asi plugin so will only work on the client side… unless you're an expert in scripts then you can try to convert it to a lua file.
