What's the fastest way to install optifine?

if double clicking the jar opens it like a zip folder, winrar or 7zip has grabbed the file type. right click it, open with, java. or open a command prompt in that folder and run java -jar with the file name.

Have you launched the matching vanilla version at least once? That's the usual reason it fails. Then download the OptiFine jar for that exact version, double click it (needs Java), hit Install, and pick the new OptiFine profile in the launcher. About two minutes start to finish. If you're using Fabric mods it won't play nicely, people use Sodium instead there.