Look on you tube for step by step tutorials on that subject. It will be very helpful. First you need to make a pattern. I use lining or very cheap fabric.
Hi Kay. My name is Cindy. If you have an old sheet, you can use it to create a pattern. Put it on your sofa and cut sections for the arms, the back, the skirt, etc. Every part needs a pattern. Label each piece as you cut it out. Once you have all the pieces cut out, lay it on the fabric and cut the fabric for each piece. Leave a seam allowance, about 3 inches additional fabric all around each piece. This will allow for sewing the pieces together. I hope this is helpful to you.
Kay: maybe these will help
Some of the pattern manufacturers like Simplicity, McCalls etc have patterns for sofa slipcovers.