I do intarsia all the time in my doll sweaters, and the most difficult part is not getting too much tension and getting puckered places or holes when you've changed color.
Practicing on a swatch is a good idea if you're just learning. I also recommend using very small balls, or bobbins, if you're using multiple colors, so you can untangle them easily - all the twisting around to prevent holes usually ends up braiding my yarn
