Temporary table postgresql function

I can't find a clear explanation of the syntax to create (and use) tables just for the inside calculations of a function. Could anyone give me a syntax exemple please ?

From what I've found, I have tried this (with and without @ before temp_table) :

CREATE FUNCTION test.myfunction()
RETURNS SETOF test.out_table
AS $$

DECLARE @temp_table TABLE
( 
        id int,
        value text
 )
BEGIN
 INSERT INTO @temp_table 
        SELECT id, value
        FROM test.another_table;

 INSERT INTO test.out_table
        SELECT id, value
        FROM @temp_table;
RETURN END
$$ LANGUAGE SQL;

I get :

ERROR: syntax error at or near "DECLARE" LINE 5: DECLARE @temp_table TABLE

-

I also tried the CREATE TABLE approach suggested here, this way :

CREATE FUNCTION test.myfunction()
RETURNS SETOF test.out_table
AS $$

    CREATE TABLE temp_table AS
        SELECT id, value
        FROM test.another_table;

    INSERT INTO test.out_table
        SELECT id, value
        FROM temp_table;

$$ LANGUAGE SQL;

And I get this :

ERROR: relation "temp_table " does not exist LINE 11: FROM temp_table

(Obviously, I'm aware the temp_table is not necessary for what I'm doing in the code above, but that's not the point :) => I want to understand the syntax to get it to work)