The bell hangs best with a seam that goes down the back of the arm. The sleeve with the pattern has an underarm seam. An easy way to move the seam is to make a muslin of the sleeve, try it on and draw a line down the back of your arm where you want the sleeve to be. End the line about one inch above the elbow. This allows for free movement of your elbow inside the bell. Cut sleeve open down the line you drew. Cut off length below the line, making sure seamlines match. You don't want one seam longer than the other! Trace new sleeve pattern, and add seam allowances to the new seamline
Determine how long you want the coat to be at its longest point. Rip or cut three panels of fabric to this measurement
Assembling the Muslin
Sew center back seam, side seam, and shoulder seams. It's a good idea to sew a sleeve in as well, though not 100% necessary. Try on. Now it's time to adjust the hem. This is mostly playing with it until you find something you like. I found that it worked best to cut the slope in just the front panels. You'll need to adjust the length on the seamlines also, since the seams are cut on the bias and therefore a little longer than the rest of the skirt.