The picture shows the top of the nettle. This is the only part of the nettle I recommend that you use and harvesting should be done with a rubber glove. Wash the tips. The amount to collect varies but about half a bucket full should suffice for a soup. There are various recipes that you could find on the BBC food website but they are all similar. Chop about 2 shallots and sauté in olive oil and a little garlic, wild garlic is also in season so that could be used instead. Add some chopped potato (2, or 1 large) once the shallots and potatoes are soft add some stock. About a litre or more depending on how much of the nettles you have decided to use. This is a basic potato soup with nettles, so if you have decided to use more of the nettles then perhaps a litre and a half of vegetable stock is advised. Chop the nettles then add into the stock and allow to simmer for about 20 minutes. Salt and pepper is added for flavour and if you are happy with the taste take off the heat and use a blender or food mixer to make a smooth soup and so avoid the stringy nettle bits.To serve I would recommend a drizzle of olive oil and or some smetana or sour cream.
A little post script about nettles. Although it is probably the scourge of some gardeners it was a vital part of British life for natural dyes and fodder for animals. Crushed it is used in cattle food and dried and powdered used in poultry food. The flowers of the nettle are loved by butterflies. So always keep some nettles in the garden as they need all the help they can get. Medicinally the Old Herbalists used it as an anti asthmatic, mainly the juice of the roots and leaves are extracted for this. The seeds have been used for stings and venoms, also as an antidote for hemlock and henbane. This information I have retrieved from a wonderful old book called A Modern Herbal by Mrs. M. Grieve. First published by Jonathan Cape in 1931. I am not recommending that you try the seeds of nettle for poisons but just sharing some ancient knowledge when there were no Glaxo Smith Kline or Astra Zenca!