Alright, first things first. If you don't have a CircleCI account yet, you'll need to create one. It's pretty straightforward. Just head over to the CircleCI homepage and sign up using your GitHub or Bitbucket account. They even have a free tier, which is perfect for small projects or if you're just tinkering around. Once you're logged in, it's time to set up a new project. CircleCI makes this super easy.

• On the CircleCI dashboard, click on the "Add Projects" tab.
• Find the repository where your Supabase project lives.
• Click the "Set Up Project" button to get things rolling.

Now comes the fun part—configuring your project. CircleCI needs a `.circleci/config.yml` file in your project's root directory. This file tells CircleCI what to do.

Here's a sample config.yml for a Node.js app:

version: 2.1
jobs:
  build:
    docker:
      - image: 'cimg/node:14.15'
    steps:
      - checkout
      - run:
          name: Update NPM
          command: 'npm install npm@latest'
      - restore_cache:
          key: dependency-cache-{{ checksum "package-lock.json" }}
      - run:
          name: Install Dependencies
          command: npm install
      - save_cache:
          key: dependency-cache-{{ checksum "package-lock.json" }}
      - run :
          name: Test
          command: npm test

In this file, the build job:

• Fetches your project files
• Updates NPM to the latest version
• Installs your project dependencies
• Saves the dependencies in a cache for future runs
• Runs your project’s tests

If you haven't already committed the `.circleci/config.yml` file to your repository, you'll need to do that now. Open your terminal, navigate to your project's root folder, and run:

git add .circleci/config.yml
git commit -m "Add CircleCI configuration"
git push origin <your-current-branch>

After pushing your `.circleci/config.yml`, head back to CircleCI. Choose your project and click on "Start Building". CircleCI should pick up the `.circleci/config.yml` file and start running your test and build jobs. For security reasons, you shouldn't add sensitive data like API keys directly in the `config.yml`. CircleCI has a secure way to handle this.

• Go to your project settings in CircleCI
• In the 'Environment Variables' section, click on 'Add Variable'
• Add all the required environment variables one by one

These environment variables will be available to your CircleCI jobs just like they are in your local development environment. CircleCI keeps them safe and encrypted.

And there you have it! Now, every time you push changes to your repository, CircleCI will trigger a build, run your tests, and give you a detailed report. You can even set up notifications to keep you in the loop on each build status.