Download the OptiFine .jar file from the official OptiFine website
Make sure you have the correct Minecraft version installed
Open the downloaded OptiFine .jar file
Click Install in the OptiFine installer window
Wait for the installation to finish
Open the Minecraft Launcher
Select the OptiFine profile from the version list
Click Play
If needed, place the OptiFine .jar into the Minecraft mods folder only when using a compatible mod loader setup