My husband did the drawing. I printed out a picture of the character and he drew a grid across it. Then, he drew another larger grid on a big piece of cardboard (using a yard stick). Square by square, he sketched the character onto the cardboard (in pencil).
When he was happy with how it looked, he traced over his lines with a Sharpie.
We painted them with acrylic craft paints and when dry, retraced the Sharpie lines for a nice sharp finish. We only used one coat of paint, and it seemed to cover just fine.
As far as time, I would say the drawing took about an hour per character. My hubby graciously did one a night over the course of a week or two. We worked together on the painting and it took us about 3 hours all together.
Large cardboard characters are definitely a low-cost way to add big fun to a party!