If you dare try to do this alone - you could start with a basic fitting pattern and cut everything with at least one inch seam allowances and extra length.  Instead of a fold down the center front cut the muslin  down the center front with an extra inch overlap like a jacket.  Now pin it together and start to fit.  Always pin the front center seam first (it is there just to get you into and out of it) mark the bust points, pin in any darts and seams.  Make sure the grain lines stay straight.   You can draw neckline and arm holes (or enlist the help of a husband or friend).  You may have to take it off and on twenty times, but it does work.  When you get everything fitting, draw the darts and seams and waistline lines on before removing the pins and cut off the excess fabric.  You will have a pattern.  Transfer it to paper and even up the darts as needed.  Then try cutting something out of another fabric - maybe with a back zipper this time.  Each time you do it you will get closer to the real thing.  Then you can compare it to a purchased pattern to see what the differences are and what changes you have to make and you will make these changes with each new pattern you try.   A sewing friend would be a  big help!