Position the ottoman against the couch. Line up the outer edge of the ottoman with the outer edge of the couch on one end or the other. Align the back edge of the ottoman with the front edge of the couch seat.
Measure the combined length of the couch seat and the top of the ottoman. Be sure to lift the back cushion of the sofa to expose the whole seat. Add two inches to this length for seam allowances if you are sewing a cushion.
Create or purchase a box cushion that matches or coordinates with the fabric of your sofa. Remove the existing sofa seat cushion; also remove the top of the ottoman if the ottoman is higher than the couch seat. Slide the new box cushion into place so that it covers the sofa seat and the ottoman top to create a chaise.