Simply download, run the JAR file with Java, and click "Install". Make sure your Minecraft launcher is closed first.
You might need to click "Preview Versions" on the official OptiFine download page to find the 1.19.1 release. Thanks for downloading Optifine 1.19.1
To use it with Forge, just drop the downloaded file into your /mods folder. Simply download, run the JAR file with Java,
If you are looking to get the best performance, here are some key takeaways from the community: run the JAR file with Java