A computer with wordprocessing software of some kind
White A4 paper
Some red paper - anything vaguely close will do
Sisors or craft knife and cutting board
Glue - wallpaper paste or watered down PVA
On the computer, create a new document in landscape A4. Using the font Arial or Helvetica at about 530 point, type the letters BRINGEMHOME or PROSECUTEBLAIR without any spaces. Before you print, set the font format to outline. If you want to be really clever you can also set a drop shadow to match the original posters.
Your document will now be 12 or 14 papges long. Print out the first half of these then put the paper back in the printer upside down before printing the rest. This way you'll have two letter on each page ready to cut out.
Cut the letters out with sissors or a craft knife. If you are a perfectionist then cut the middle bar on the letter E's so it's a bit shorter to match the ones on the poster. Also cut out any punctuation you think you need; apostrophe and full stop.
Now get your red paper. This could be six or seven sheets of red A4 paper glued together in landscape with about 1cm overlap, or perhaps roll of some other suitable paper.
Arrange the letters on the red paper in the correct order paying special attention to the spacing. Each letter should be about about half an inch apart. Spacing between words is a little less than the width of a letter. When it is all correctly aligned on the paper, glue the letters down making sure the edges are stuck down well.
When it was completely dried, roll the whole thing up from the last letter so the letters are inside the roll. This way it will unroll the right way round when you start after the words, FOR THEIR SAKE,
You should reccy your chosen location before you actually put up your subvert and plan your route there and back. Check for CCTV cameras and dress approbriately. Think about where you can be seen from, both passing cars, pedestrians and overlooking windows. Will you need something to stand on like wheelie bin or a pallet, if so, think about how you will get it there and what will you do with it afterwards.
You'll also want the help of some trusted friends althought it is possible to do it alone. It makes it much easier to have two or three people doing it, one holding and unrolling the paper, one person pasting and another stabalising whatever you bring to stand and keeping a look out. Ideally you'd have a couple more people as look outs further up and down the road with some means to communicate with you such as walkie-talkies.
When you are done and safe at home, consider going back in the morning to casually take a photo and perhaps anonymously inform your local paper.
Since it's become something of a national story you'll probably want to make sure you or your mates don't go around telling people you are responsible.