In reply to Y Gribin:
Presuming you are starting from the summit? Scramble down the South Ridge with care to the col between Tryfan and Glyder Fach - turn West and the easiest way is to follow the well defined path down past Llyn Bochlwyd and on down to Ogwen, then walk back along the road.
From the col there is a less well defined path which follows the slope round towards the eastern end of Llyn Ogwen but it is less defined lower down and you tend make your own way down.