As I mentioned before, making a slipcover for a sofa or stuffed armchair is a bit more involved. The cover is fitted by placing your entire fabric piece (minus any yardage you will be using for cushion slipcovers) over the sofa or armchair; you then tuck the fabric in and mark where you will make pleats and measure for the hem.
If you are like me, this procedure is difficult to visualize. Where do the pleats go? That is why I suggest you refer to a book if you want to make a sofa or armchair slipcover.
The following books give clear, step-by-step illustrated instructions for making slipcovers. They are fairly inexpensive at Amazon and you may be able to find them or similar books at your public library.
The Complete Photo Guide to Slipcovers: Transform Your Furniture with Fitted or Casual Covers by Linda Neubauer
Custom Slipcovers Made Easy: Weekend Projects to Dress Up Your Décor by Elizabeth Dubicki
Slipcover Style by Alison Wormleighton (under $5 at Amazon)