• First things first, sign up at Supabase.
• Create a new project. You'll need to fill in some details like the name, region, and a database password.
• Make sure to jot down your API details (Public API URL and anon key). You'll need these later.

• Head over to the "Database" tab.
• Create new tables and schemas based on what your project needs.
• If you need to, go ahead and populate those tables with some initial data.

• In the "Database" tab, find and select "Functions".
• Click on "New Function" and give your function a name.
• Write your SQL-based function in the SQL editor. Here's an example:
  ```sql
  CREATE OR REPLACE FUNCTION get_user_posts(user_id UUID)
  RETURNS SETOF posts AS $$
  BEGIN
  RETURN QUERY SELECT * FROM posts WHERE posts.user_id = user_id;
  END;
  $$ LANGUAGE plpgsql;
  ```
• Don't forget to save the function.

• Supabase automatically creates RESTful endpoints for your functions.
• Check out the "API" tab to see the auto-generated documentation for your functions.

• Go to the "Auth" tab to set up API security settings.
• Define roles and permissions for accessing the API.
• If needed, apply RLS (Row-Level Security) policies:
  ```sql
  CREATE POLICY "Only user can select their posts"
  ON posts
  FOR SELECT
  USING ( auth.uid() = user_id );
  ```

• Use tools like Postman or cURL to test your endpoints.
• Send a GET request to your function endpoint:
  ```
  GET {YOUR_SUPABASE_URL}/rest/v1/rpc/get_user_posts?id=user_id_here
  ```
• Include the API key in your header for authentication:
  ```json
  {
  "apikey": "",
  "Authorization": "Bearer "
  }
  ```

• Use the Supabase client library in your application.

• Initialize the library:
  ```javascript
  import { createClient } from '@supabase/supabase-js';

  const supabaseUrl = 'https://xyzcompany.supabase.co';
  const supabaseKey = 'public-anon-key';
  const supabase = createClient(supabaseUrl, supabaseKey);
  ```

• Call your custom function through the Supabase client:
  ```javascript
  const { data, error } = await supabase
  .rpc('get_user_posts', { user_id: 'user_id_here' });

  if (error) console.error('Error:', error);
  else console.log('Data:', data);
  ```

• Use the "Logs" tab to keep an eye on API usage and errors.
• Adjust configurations and permissions as needed based on log data and user feedback.

By following these steps, you can create, test, secure, and integrate custom APIs with Supabase functions seamlessly into your application.