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To combine data from a joined table using PostgreSQL aggregate functions, you can use the following steps:

1. Join the tables using the appropriate join syntax (INNER JOIN, LEFT JOIN, etc.).
2. Group the results using the GROUP BY clause and specify the column(s) you want to group by.
3. Use the appropriate aggregate function(s) to combine the data from the joined table, such as SUM, AVG, MAX, MIN, COUNT, etc.
4. Optionally, you can use the HAVING clause to specify conditions for the groups.

For example, suppose you have two tables: customers and orders. You want to find the total amount of orders for each customer. You can use the following query:

SELECT customers.name, SUM(orders.amount) AS total_amount
FROM customers
INNER JOIN orders
ON customers.id = orders.customer_id
GROUP BY customers.name

This query joins the customers and orders tables on the customer_id column and groups the results by the customer's name. The SUM function is used to calculate the total amount of orders for each customer. The resulting table will have two columns: the customer's name and their total order amount.