ChainSwordCS

The OptiFine installer requires that MC be installed in the default launcher and has opened the version of MC at least once.  Do this first.

1. Download OptiFine
2. Extract OptiFine 
  a. Double click the download from step 1
  b. Click Extract button
  c. Navigate to MultiMC instance you wish to install OptiFine into (C:\mc\MultiMC\instances\1.13\.minecraft\)
3. Move extracted Optifine to instance \ libraries folder (C:\mc\MultiMC\instances\1.13\libraries\)
4. In MultiMC edit instance, go to Version screen and select the Minecraft entry in the list.  Then click Add to Minecraft.jar
5. Select the OptiFine file you extracted and moved in step 3.