Now that you know what size you need, measure it out on the fabric with one long side on the fabric fold, using the yardstick to make straight lines to cut on. Try not to pull on the fabric too much as it will easily stretch out of shape - using a sharp pair of scissors really helps.
Make sure that you cut the fabric with the grain going across - in other words, you want the fabric to stretch from side to side, not from top to bottom. This is important - the the side-to-side stretching will give you a nice fit.
Cut out your fabric.
Before you go any farther place a pin in the cloth at the top of the fabric fold - this will let you know where the center of the shirt front is and will help later.