Pattern for a Fitted Doublet
Modern commercially available clothing patterns aren't usually the right shape for adapting to make medieval clothing. Dressmaker's Dummies can make life a lot easier if you're planning on making a lot of close fitting medieval clothing, although they are expensive, and men's ones can be hard to find.
For a cheaper alternative you'll need an old T-shirt, or similar, a roll of duct tape and (preferably) someone to help you.
Put on the T-shirt and wrap the duct tape around yourself, over the shoulders and so on, so as to form a cast of your body. It's easier if someone else can do this for you.
Mark on this cast where the seams should be for your chosen style of doublet. Cut through the cast along the lines. The resulting pattern will be slightly three dimensional and curved, but should be relatively easy to use
A pattern for a later medieval corset can be found at http://www.dnaco.net/~aleed/corsets/pattern.html corset pattern