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To assign the return value of a SQL query to a psql variable, the following process can be followed:

1. Declare the psql variable using the command \set variable_name value.

2. Write the SQL query to retrieve the required data.

3. Use the SELECT statement to execute the SQL query and assign the result to the psql variable.

4. Use the :variable_name notation to reference the psql variable in subsequent commands.

Example:

-- Declare psql variable
\set customer_count 0

-- SQL query to retrieve customer count
SELECT COUNT(*) FROM customers;

-- Assign result to psql variable
\set customer_count :SELECT

-- Use psql variable in subsequent commands
SELECT * FROM orders WHERE customer_id = :customer_count;