Well... slipcovers don't necessarily follow the same lines as upholstery. Upholsterers can use tacks and staples to defy gravity; slipcover makers can't, and must let fabric have its own selfish way. :D
I would pin in one single welted horizontal seam across the top, and a vertical welted seam down the middle. I would gently gather this vertical seam at the top to separate and define the left and right sides. I wouldn't worry about the top and bottom separation.
I would pin in one single welted horizontal seam across the top, and a vertical welted seam down the middle. I would gently gather this vertical seam at the top to separate and define the left and right sides. I wouldn't worry about the top and bottom separation.