To be honest you realy need to get a workshop to press these bearings out and press the new ones in, it is beyond the diy mechanic to do this without probably damaging something.

The following advice applies if you have drums at the rear - not sure how much of it applies to rear disks...

To change the rear bearing, you'll need to undo the rear hub-nut - the drum/hub and bearing will then come off together which will allow you to take it to a garage and get them to press the old bearing out and new one in. I found a friendly local garage who did this for me for a tenner.

A word of warning about the hub-nut - it will be done up extremely tight (235nm according to this thread : http://www.honestjohn.co.uk/forum/posts/?t=81562), so you'll obviously have to re-tighten it to this torque setting, meaning you're going to need a long-handled torque wrench (min. 2ft I'd say) and a tight-fitting socket.

I fitted cheap (~£20) UniPart bearings to my Focus and they lasted <18 months IIRC so even though they're twice the price, I would recommend using a Ford bearing. The corners on the UniPart hub-nut also started to round off before the correct torque was reached when refitting, so I would would re-use the Ford nut provided it's not been on and off too many times (I think Ford recommend 3 times max).