I'll try to explain.
your mouth is open
teeth aren't closed, too
the tip of the tongue is on the alveols
on the pronouncing it the airstream must be very strong. mm you may get this sound pronouncing [ж] but airstream much stronger, it sounds like carmotor). May be, it will be easier for you if the sound [т] stands before it. For example: трын-трава, трактор, трон.. or even - трок, трыба, трана )).
Some people offer one method: put your index finger between the teeth, pronounce [ж], and on making it move the finger right and left. I don't know if it works).
May be the following link is useful: http://logoped.ru/zv-r02.htm