Measure the shirt to fit your doll using the doll clothes tutorial.
Draw the shoulder areas where you plan to cut, and make tabs in the shoulder areas that you will keep. Add whatever details you like - I put in a keyhole neckline and drew stripes.
Color it in, cut it out, tape it, and you're done! The tabs are important because they keep the top of the shirt from looking too "open." If you're a real stickler for proper clothes fit, make tabs for the sleeves too.
What's your favorite way to beat the heat?