Don't leave any stem above a bud, or protruding from a main stem when pruning, as this will die back and is open to infection by diseases such as Coral Spot . Start sowing seeds indoors for the coming season. If planning a herbaceous border, raise many of the plants from seed, they will be ready for planting out in the autumn and some may even flower this year. Growing potatoes requires quite a lot of space and work for a product which is available at low cost. Also they mature before the risk of blight would require spraying.
Seed potatoes are available now, buy only certified tubers.
Place the cut side down and watch out for rot. If you can choose, pick egg-sized ones. If you're preparing a new bed along a boundary or at a fence next to rough ground, why no insert a barrier to prevent the ingress of creeping roots. Use some 1000 gage plastic sheeting available from builder's suppliers. Cut strips about 30 to 40 cm wide and bury them about 20 to 30 cm deep, turn a 5cm horizontal edge at the base to act as an anchor.