You don't have to be an expert knitter or crocheter to yarnbomb; a knowledge of the basics is all that you'll need.
There are two ways that Knitorious 209 approaches yarnbombing. One way is to pre-make little items using scraps of yarn: flowers, butterflies, pom-pons... quick and easy projects that can take only a few minutes to do. Add a knit or crochet strap or attach to a larger piece, like a rectangle. This way, you can always carry some with you in case you find a spot in need of a little color.
Many of our patterns have been found here: Crochet Pattern Central and Knitting Pattern Central. We've also free-handed many items (crochet works great for this, but knitters can do it, also). If you want a little variety, check out different stitch patterns. Granny squares, basketweave stitch, puff stitch... These all add a bit of texture and interest.
The other way we've yarnbombed is pre-planning and designing, like we did with MLK Jr. Plaza. I used Google Maps to find the place, then switched to street view to get an idea of the place and the neighborhood around it. What kind of fixtures were there: statues, monuments, light poles, benches? What kind of plant life did we have? Was there anything else interesting nearby?
After that, we actually drove over there (in a light rain) and three of us wandered around the park after dark; brainstorming. We had tape measures to get some measurements of parking meters, benches, flag pole, statue. We eyed the trees and how low the branches were, and looked around, watching for pedestrian and street traffic patterns.
The evening of, we dressed normally; nothing bright or flashy. Purses were not brought; just car keys and ID, zip ties and round-tipped scissors (in case we get caught and have to take things down), and dark tote bags with the "bombs" inside, separated into their own plastic bags so nothing will tangle. (This is crucial with items like the pom-bombs.) It is quick and easy to use a zip tie or two to secure a yarnbomb. Some people sew or crochet together the ends of the item - that can be difficult in the dark! There are people who yarnbomb in broad daylight... so far, we haven't tried that option. :)
This weekend, I'll post the instructions for how some of us make our pom-bombs! We use fancy pom-pon makers, but you can make your own out of some cardboard!