For more information, check http://www.yarnbombing.com, a blog devoted to yarn bombing. There is also a newly published book on the subject called Yarn Bombing: The Art of Crochet and Knit Graffiti by Arsenal Pulp Press.
Step 1: Prep and Knitting
If you want your tag to look the best, take measurements of what you're going to tag first. You can then use newspaper to cut out a template. When you finish a piece of yarncraft, you can lay it down on the template and see if it will stretch to fit, or if strips or squares need to be knitted up and sewn on to give the tag the proper shape and size.
Then, knit. I like to use simple light post tags to experiment with stitch patterns I've come across. This week I played with diamond and ripple patterns, as well as a thin strip of basketweave stitching.
Step 2: Getting Ready to Release Your Knits
After attaching the paper tag, one thing that you will find that makes the while process much faster is to prep the piece for putting up. After finishing, I always leave a long tail on the piece and pre-thread it with a large tapestry needle. That way at the site I only have to take the tag out of my tote and there's already a needle and thread attached so I can get right to sewing! Just be sure to remember your needle! You don't want to leave it behind!
Roll the tags up and put them in a tote along with extra needles, scissors and your camera!
Step 3: Yarn Bombing: The action!
Wrap your tag around the target and sew it up with the pre-threaded yarn tail. When you've tied it off, snip off the excess and voila! You've just installed knit graffiti With a snug fitting piece, using bobby pins or clips to hold the piece together down the seam while you work can be a real lifesaver. After it's sewn, take a minute to pull it into the place you'd like it to be for presentation and step back to admire your work.
Using bright colors will really make your tag 'pop'. Remember to be considerate. Don't tag over important signs or across spaces that would obstruct foot or motor traffic. If you are stopped and questioned by the fuzz, be polite! And it might help not to call it yarn bombing or knit graffiti. "Urban beautification project" sounds a little better. ;)
Some pieces will stay up for a year or more, and others will only last hours. That's part of releasing the knits, and graffiti in general. You don't know how long your art will be there, but it is there for the public to enjoy. One concern with knit graffiti is smell/mold caused by the weather. If there has been a lot of moisture and rain lately, you may want to go back to your knits and cut them down if they're getting nasty. When you see your tag has been removed, don't get disheartened. It may be making someone very happy by adorning something in their home, hanging on a wall or even just transferred somewhere else in the city. And you can always craft more. ;)