okay, no takers, but we will procede like we are useful.
The next step is to visualize where you'd predict or like the seams to be. The seams must be placed where you want the cords to appear. Seams must also join areas where grain direction or pattern direction would ideally change.
Additionally, seams protect areas of stress or shaping that would tear if only one piece of fabric was asked to make severe turns. Seams shape, seams atract attention, seams hold zippers.
Seams ( cords ) create a new sillohuette ( sp?? ) . Seams need not, and in many cases cannot, fall where the upholsterer placed the cords or joins. We have no nails, no glue. Simple gravity and tension are at work, we manipulate but not force.
The first cuts of fabric ( all body parts ) must then have a margin of error allowed for adjusting the placement. I like cuts three inches over my hoped for seam placement, so that I may change the plan as I see a better answer ( or a neccessary answer even if I don't like it ).
How does a beginer answer this question for the first time ( where seams go ? ). Look at existing slipcovers. Magazines yes, but in the home in use as well.
Second suggestion, place your hands on the furniture as you look at it. Interlace your fingers and form right angles with fingers. Hold hands so that the fingers and palms form north south east west positions. Find corners on the rounded surfaces of the furniture. Some corners are obvious, and probably needed seam location. There may be more seams needed, on these more rounded surfaces. Hard corners are required seams: rounded corners are possible seams. A seam may be a choice, or perhaps a more practical answer than making one piece of fabric bend around a corner.