Find a big enough shirt that between the sleeve seams will accommodate the size of shirt you want and with a neck size you already like. Lay it flat.
Lay a shirt you want to use as a pattern on top of the larger shirt.
Draw around your pattern shirt leaving 1/4" or 1/2" for seam allowance. It's always better to go bigger first because ya can't go back if it's too small.
Only cut along the lines you traced.
Turn inside out (or leave right side out if you want the stitching to show), line the edges back up, and sew from the edge of the sleeve all the way down to the bottom of the shirt making sure to reinforce the stitches at the beginning and end of your seam so they don't come undone.