Setup

Create a folder with a single file in it, called meta.json.

In this meta.json file, enter the following:

{
  "schemaVersion": 1,
  "hotfixesFormat": null,
  "versions": [
 
  ]
}

The folder should look like this:

Host the files on a file-hosting platform

If you cannot find a suitable hosting platform, you can upload the files to a GitHub repository.

In Minecraft, install Modpack Update Checker

Modpack Update Checker can be found on Curseforge (opens in a new tab) and Modrinth (opens in a new tab).

Once Minecraft loads with Modpack Update Checker installed, a file in config/modpack-update-checker/config.json should appear.

The file's contents are the following:

{
  "schemaVersion": 6,
  "currentVersion": "1.0.0",
  "modpackName": "Modpack Name Goes Here",
  "modpackAuthor": "My Modpack Studio",
  "modpackReleaseType": "stable", // Valid values are [stable, beta, alpha]
  "githubRepo": "https://raw.githubusercontent.com/Jab125/TestModpackUpdateChecker/simple/",
  "advanced": {
    "bccEnabled": false,
    "expandButton": false,
    "updateCheckerType": 2,
    "addButton": true,
    "mainMenuCreditsIntegration": true,
    "modpackMods": []
  }
}

(Comments aren't supported in JSON, remove them beforehand.) Everything advanced is optional, and not including them will make them use their default values (which are shown above).

Edit config/modpack-update-checker/config.json

Replace modpackName with your modpack's name, modpackAuthor with the author of the modpack.
Replace githubRepo with the base url of your aforementioned folder. THE URL MUST END IN A SLASH! (Despite the field being called githubRepo, it accepts any url.) Replace currentVersion with the current version of your modpack. The current version will be compared with the latest version when Minecraft starts.

Edit config/modpack-update-checker-info.txt (create the file if it doesn't exist)

This file contains plain text, and serves as a way to give out basic information about your modpack. Ingame, lines that are too long get trimmed, and urls are clickable. (markdown urls are also supported!)