Firstly, you will reach the metro stop adjacent to the Azadi Stadium, as you say, through Line 5, which is called Golshahr. You will most likely get to Sadeghiyeh station, which is the final stop on Line 2 in the western direction, by metro, and the platform you need next is right on your right hand side as you exit the Line 2 train. Look for signs saying “Golshahr”, as that is your line (Line 5). After two stops you will reach Varzeshgah-e Azadi station, which is the one nearest to the huge national stadium. However, you cannot walk there in a straight line. Turn right as soon as you exit the station, and 200 yards further down you will see small shuttle buses, which are used for bringing people from the station area to the stadium entrance. It will cost you the equivalent of 6 US Cents (!) (500 Tomans as of August 29).