Vector illustration workflow
I finished the posters. I chose simple vector illustrations and sweet/retro colour palette. To my surprise, the ones that looked best on screen, were the weakest in print, and the one that I liked the least is now my favourite after I saw it printed off.
I recorded my workflow from creating one of the illustrations:
First, I sketched a few ideas and thought how to communicate the idea. I also had to think which one of them would work best with the technique I wanted to use. My sketches are in this post.
Then I scanned the sketch to my computer, lowered the opacity a bit and locked it in the background. I'm using Illustrator CS6 with snap to grid turned on. This is what my workspace for vector illustrations looks like (yes, I stick it to the left side!):
I recreated the sketch by drawing simple shapes on it and moving them to the side. I only used the ellipse, rounded rectangle, rectangle and polygon tool, occasionally adding anchor points and moving them around with the direct selection tool. The pathfinder panel is your friend with drawings like this one. The candy floss (and the tiny writing on the backpack) was created with the blob brush tool. You could, of course, trace it with the pen tool, but this shape didn't have to be precise and the blob brush saves lots of time.
Somewhere along the way I decided to move the candy floss to the other side and turn the kid's head towards it.
I also created a colour palette, using the Kuler extension and colour guide panel. Creating groups of swatches from selected art also helps a lot when you're working with colours. Then I just start colouring by selecting the shape and copying the colour from a square beside it (V - Select, I - Eyedropper, click on square, repeat 51454512 times)
I created a few versions of the picture before deciding on the final one and adding details, gradients and shadows.
Then I just add a few subtle (!) gradients. Use gradients sparingly, otherwise it will look like from the 90's (and that's not the kind of nostalgia we're trying to achieve here).
Final illustration, as it appears on the poster: