Head over to the Supabase dashboard and pick your project. Then, navigate to the "Auth" section and click on "Policies". Here, you'll want to create a new Row-Level Security (RLS) policy for the table you want to lock down.

In the table's settings, look for the "RLS" toggle and switch it on. This makes sure all queries go through the RLS policies you've set up. If you don't enable this, the policies won't do anything, even if they're defined.

Define your policy conditions using SQL. For instance, if you want only the owner of a row to access their data, you can create a policy condition like this:

(current_user_id() = user_id)

Here, current_user_id() is a function that returns the ID of the currently authenticated user, and user_id is the column in the table that holds the ID of the user who owns the row.

Apply the policy by inserting the SQL command into the SQL editor in the Supabase dashboard. For example:

CREATE POLICY "Select own rows"
ON public.your_table_name
FOR SELECT
USING (current_user_id() = user_id);

This policy lets users select only their own rows from the your_table_name table.

Run some queries to make sure the policy works as expected. Log in as different users and try to access rows they don't own. Check that they can't see data they shouldn't be able to access.

If you need multiple types of access (select, insert, update, delete), create specific policies for each. For example, for an insert policy:

CREATE POLICY "Insert own rows"
ON public.your_table_name
FOR INSERT
WITH CHECK (current_user_id() = user_id);

This ensures that users can insert rows but only with their own user_id.

Regularly review and update policies as needed to keep up with changes in your app's security needs. This might mean adjusting conditions, adding new policies, or getting rid of outdated ones.