Step 1: Measure, Rough Cut, Pin Welting: The front of the rolled arm is the most noticably detail of a slipcover, so start there to get it to fit there first. Pic 1A. Measure the height and width of the arm front. Pic 1B. Add enough allowance on all edges (an inch on each side and 3 inches in length). Cut a rectangle this rough size. Pin it to the arm front with the RIGHT side of the fabric facing out. Pic 1C. Pin the welting to the arm edges to EXACTLY track the edges of the arm roll. You are pinning this welting to the RIGHT side of the fabric. It will stay pinned all the way over to the sewing machine.
Step 2: Remove the rectangle and trim: Pic 2A. Remove the pinned rectangle. Pic 2B. Trim the edges.
Step 3: Machine baste the welting to the arm front, carefully following the curve.
Step 4: Test fit the basted piece, adjust if necessary.
Step 5: Create a mirror image for the other arm: Assuming you have a symetrical couch, make a mirror image.
Step 6: Rough measure and drape: Measure and cut a rectangle to go up and over the arm, from the floor to the platform. Add an inch allowance on each edge. Drape it over the arm, with the WRONG side facing out. Pin it tightly onto the arm. Pin the curvy front piece onto the arm, with the WRONG side facing out.
Step 7: Pin arm front to arm side: Make it tight and tidy. Pin in the seam allowance. Expect lots of excess seam allowance.
Step 8: Trim away excess seam allowance.
Step 9: Sew the seam. Take the pinned pieces off the couch and sew them, being careful to sew tight to the welting without getting puckers in it. (I DO get puckers. I then unpick and resew... I'm improving tho).
Step 10: Test fit the finished arm. (Note that during the pin-fitting with WRONG sides out, the pieces were fitted on the LEFT arm of the couch. Now the finished arm, with right side out belongs on the RIGHT arm of the couch).
Step 11: Repeat for the other arm, flipping left and right.
One more Detail: Putting high heels on my loveseat: My loveseat is old. Like most older style couches, it sits low (cushion bottom is 11 inches off the floor). I wanted it 2 inches taller, like newer couches often are. So Mr. Summer Gypsy screwed in 2x4 to serve as risers on each of the legs to raise it.