To install a new instance, go to the releases page and select the optifine version you want to use. Download or copy the link to the zip for the version you want (e.g. 1.14.4.zip). You can then import the zip file in MultiMC to create an instance with OptiFine.
With each release since 1.14.4 F3 pre2, a patch zip will be included. This only includes the mmc-pack.json, libraries folder, and patches folder. You can download this zip file and extract it to your instance folder. Your instance's .minecraft folder, pack icon, and instance config will not be removed or changed.