Not hard to do, I use Girling seal kits and obtain stainless steel pistons,I split the caliper for ease of working,(carefull not to loose the seal from the join,its not included in the kit) remove the old seal and wiper seal and retainer,spend a lot of time getting the grooves perfectly cleaned out,a small right angled screwdriver is handy,lubricate all seal parts with Lucas red brake grease,insert the main seal,insert the piston half way in,insert the wiper seal inside its retainer ring,fill the groove in the wiper seal itself with the red grease,place over the piston and slide down,now for the tricky part,very easy to distort the retainer,I use a steel disc,big enough to cover the retainer,carefully tap the piston down,the disc eventually taking the retainer in as the piston keeps it square on to its groove. Suffice to say,keep as many of the origional retainers reusable as spares,they do come in handy. Replace all the pistons and seals,reassemble the two halves and the jobs done. Just make sure before starting all this that the bleed nipple actually unscrews,it would be bad day to have a newly rebuilt caliper and no means of bleeding it!
