Clean your love seat or sofa thoroughly. Vacuum under the cushions and into the corners, and shampoo the furniture with upholstery shampoo. Not only will your furniture smell better, but a thorough cleaning removes dirt which can act like an abrasive to wear down the fabric.
Place quilt batting lengthwise on the floor.
Center your loose back pillows over the quilt batting.
Wrap your pillows in the length of batting to create one long pillow. Fold the batting over the pillows and use safety pins to attach the batting to itself along the top edges and where the long sides overlap.
Roll a section of quilt batting into a roll and stuff the batting into the crease between the bottom cushions.
Cover the bottom cushions with quilt batting. Tuck the batting deep into the back crease and along the sides. This will smooth out the bottom cushions and make them look like a single cushion.
Pre-wash and iron your slipcover.
Place your bundled single back cushion into place on the furniture.
Position the clean and smooth slipcover over your couch or loveseat, following the manufacturer's directions. Tuck excess fabric into the crease between the seat cushion back and bottom cushions with a spatula. Smooth out the fabric, working from the center out.
Tuck a pool noodle deep into the bottom back crease to hold the slipcover in place. Use rolled magazines tucked deeply along each arm crease. The magazines and noodle hold the cover tight against the cushions and frame.
Check your slipcover's hem all around to make sure it is even and level, and add accent pillows to dress up the cover.