First, you need to create a free Userbase admin account. No credit card required.
Then, you need to include the Userbase SDK in your web app.
You can either include the SDK with a <script> tag:
Or, you can use the npm package:
npm install userbase-js
You can check the browser compatibility list here.
In your admin account, you will find a Starter app. Get its App ID, and initialize the Userbase SDK with it.
And you're all set. You can now proceed to the SDK section, or jump straight to the Quickstart to see how to build a very basic web app in a single HTML file.