Determine what colors you will use to decorate your window seat. Look at the colors already in the room. Select colors that complement the color scheme already in place.
Purchase a cushion runner (a long bench shaped cushion that will run the length of your window seat), throw pillows and blankets that fall within the color scheme you've chosen.
Sew at least four Velcro pieces (possibly more if your cushion is longer) to the bottom of the cushion runner. Fix the same number of Velcro pieces to the seat itself in places that lines up with the Velcro on the cushion runner. Secure the cushion runner to the window seat.
Place the throw pillows strategically on the window seat--because it is a bay window, you can use the extra space created where the window panes curve out to pile more pillows. Place two larger pillows on either end of the bench. Place two smaller pillows leaning against the two larger pillows. Throw any smaller throw pillows you have in multiple places across the window seat.
Fold a blanket that matches the color scheme you've selected for you window seat decorations. Place the folded blanket strategically on the window sill, or to one side or the other of the bench. Drape the blanket attractively over the window seat for a more inviting and carefree look.