All credit goes to the original mod developers!

A full list of included mods (except Optifine) is in *.zip/modlist.html.
Each mod listed has a link to its CurseForge page.
Optifine can be found here: https://optifine.net

INITIAL INSTALLATION:

Curseforge:
Download the .zip file
Click the Import button on the Curseforge home screen, select the .zip file you downloaded and click import.

ATLauncher:
Download the .Zip File
Go to the Instances Tab in ATLauncher and click the Import button in the top right.
Click 'Browse', Select the .zip and click import.
OR
right click the .zip file on the website and click 'copy link' or 'copy web address'
Click the Import button the Instances tab in ATLauncher, paste the link/address you copied into the URL box and click import

Modrinth:
Download the .mrpack file
Click the '+ New Instance' button in the bottom left and import the .mrpack file.

Minecraft: just- download ATLauncher- It's better. trust me.

UPDATE EXISTING INSTANCE:

Copy 'saves', 'options.txt' and 'optionsof.txt' from your existing instance to your desktop
remove/delete the instance from your launcher
reinstall the pack using the steps above
run the fresh install once to complete first time setup and then close the game once fully loaded
open the folder for the new instance, and then delete 'saves', 'options.txt' and 'optionsof.txt'
replace the above files/folders with the ones you copied from your previous instance.