Good morning Col :D
Pretty easy to do mate, the 1st thing i always do is to give the calipers a good rub over with a wire brush to remove most of the brake dust/debris that collects on the calipers, then i usually spray them with a brake cleaner/white spirit to give a clean surface to paint on, And you can mask the surrounding areas around the caliper if you want to, but there isn't much there that needs covering up and if you do get any paint on the disc it will soon come off when braking,
Using a small brush (1 inch/1/2 inch) apply the paint to the caliper in a good even coating, try and avoid the nuts, bolts and clips if you can mate, (just good practice not to paint them) and let dry,
I usually paint 1 and then refit the wheel then move onto the next one, and by the time you have done all 4 the first one will be pretty well dry, with the hammerite i've only ever done the 1 coat and they seem to come up quite well, but its up to you if you want to do another coat?
Some guys take the calipers off to paint, but i find it easier to do them while still on the car and it lets you refit the wheel so you don't need to handle them while wet :)