How to: sew a duvet or coverlet without a center seam
Lay out your fabric (blue) and cut 1 piece (per side) the full length of the comforter you are covering or the full length of the bed (and then some) if you only intend to make a single sided bedspread. Usually this length measures about 75-80″ long.
Take the half sized pieces and place them on either side of of the full size center piece, sew the pieces together. Depending on the bed size, you will need to trim the side pieces to the correct overall width you are looking for. Repeat these steps if you are making a duvet and need a front side and a back side that will be sewn together.
Check out the full tutorial- making a duvet with a buttoned closure, in real pictures, step-by-step.