Use a "C" clamp.
Turn the piston clockwise and it scews itself in...
With an Easy-Out.
7mm
it takes a 7mm Allen wrench to remove calipers on 2003 ford focus.
Unlike the front calipers, pistons in the rear usually have to be rotated to compress them.
7mm Allen
Metric Allen wrench.
Remove the wheels, then the bolt that holds the pin that holds the caliper on. Drive the pin out then depress the piston completely in the caliper. Make sure to use the anti squeek grease or shims when reassembling it.
Your local motor spares shop should sell a Draper caliper retraction tool. Mine cost around £12 and does the job perfectly.
Yes, you need to remove the caliper first, then use the old brake pad and a C clamp to push down the piston of the caliper.
Use a large C- clamp to depress the piston enough to remove the caliper.
They're metric. Try them until one fits, that's what I do.