I probably did this in entirely the wrong way! I'm a huge fan of Adobe Illustrator and use it frequently at work. Sadly it's a bit expensive for me to get a copy for my creaky old MacBook at home. I tried Inkscape, a decent free alternative but, although I can see it has potential, I had a lot of trouble getting used to the interface. (I've been using Adobe products for the best part of 20 years, so my mind has trouble using non-Adobe keyboard shortcuts!)
So, I just ended up doing this with the lasso tool and layers in Photoshop, simple as that. I had to use a photo of Colin and an Obama poster for reference. If you zoom in veeeeeeeery close, you might be able to see that some edges aren't as smooth as vectors.