I made a paper pattern and using the floor as my work table, pinned the pieces to the fabric and cut. (This is the only pix as I was working...I really got into it and forgot about the camera).
Here's the finish! Pretty good! (OK, I was very smart or lucky and all the wonky iffy spots are covered by the seat cushions!)
I really like it-I like the relaxed (wrinkly) look of it. The throw pillows show the original upholstered fabric (which we are planning to change-DH is going to the US soon and will return with new fabric and then we'll have the sofa professionally reupholstered but I may still use my slipcover during the summer months-I really like it!).
So if you have a chair or sofa that needs a new look...try slipcovers. If I could do it, so can you!