hints on Paschimottanasana

Hi everyone,
I am new to yoga – but a keen and eager learner. I have had a back problem (i.e. shoulder hurting and lumber region hurting) for a very long time (we are talking years here). Having done a few cleanses my shoulder pain is nearly gone but my lumber region and hip pain persists. I would like to incorporate a yoga asana, which would assist in healing my lumber/hip region problems. I understand that Paschimottanasana may be suitable. However it looks soo easy in pictures – but when I try to do it myself it is not that easy. I am barely able to reach my ankles let alone clasp my hands around the feet. I wish I could do that but I cant. Anyone please any hints on how I might go about getting there. BTW I am learning from Iyengar’s Light on Yoga.
Thanks. Best Wishes to all
StandFast.