This guide will give you an idea as to how much stripping down is needed to get to the wheel cylinder.
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In addition, the brake flexi-pipe needs disconnecting from the cylinder and the cylinder unbolted from it's mounting.
You'll need new brake fluid and to bleed the brake line when reassembled.
The hub nut needs to be tightened to 235 Nm, and preferably should be replaced.
Assuming that the cylinder is leaking, brake fluid may have contaminated the shoe linings. If so, you should replace the sets in both hubs. There were two sizes of shoes fitted to the Puma, depending on year of manufacture, and I'm not sure, but there may have also been two different cylinders.
Get yourself a Fiesta manual for step-by-step instructions.
Note:
The proper way (Ford), is to undo the four bolts at the rear, which releases the hub - the hub and drum coming off as an assembly without disturbing the hub nut/bearing. The main problem with this method is that the bolts are normally rusted in (I managed to undo mine and used new bolts to reassemble).