Don't toss those old cushions; just make slipcovers for them. Once you get the hang of it, you can make a custom slipcover for any size and shape cushion. For simplicity's sake, start here with a round cushion. You can design custom slipcovers from a variety of fabrics. Dye the fabric any way you like before making the cover, or add decorations when finished if desired.
Measure across the diameter of the cushion. Measure all the way across the cushion at the widest part, from the bottom of the left side, across the top of the cushion, to the bottom of the right side. Write down the measurement.
Add two inches to the measurement for a small cushion, up to five inches for a large cushion. This addition is to allow for the seam allowance and gathering. You want the slipcover to extend over the edge of the cushion and extend part of the way across the bottom of the cushion. Cut the muslin to those dimensions. It does not need to be a perfect circle, but make sure the fabric is round.
Drape the muslin over the cushion, smooth it across the cushion, and gather the excess at the bottom. How much excess fabric you need will depend on the size of the cushion. Gauge whether or not you have an acceptable amount of fabric to wrap around the cushion and be hidden under the bottom of the cushion when the elastic is in place. The case for the elastic will take up about two inches of the fabric, so if it seems as if there's about a couple of inches too much, then it is just right. Trim only if needed.
Iron flat the fabric that you're using for the slipcover. Set aside the iron but leave it on, as you'll use it in the next step. Cut the fabric to the same size as the muslin.
Fold the edge of the fabric over 1/2 inch and iron it flat. Ironing a hem in a circle requires snipping some tiny triangles of fabric out of the hem in a few places so that it will lie flat. If your slipcover needs to lie flat, cut the fabric in a few places around the circle. If it doesn't matter, skip the snipping.
Fold over the fabric edge again, this time making the fold one inch, and iron it flat.
Use the sewing machine and regular thread to put a line of straight stitches down the left side of the folded-over edge. This will form a pocket where the elastic will go. Stop machine stitching two inches from the starting point, leaving room to insert the elastic. Cut the threads and remove from the machine.
Put the safety pin into the end of the elastic, and fasten it. Begin threading the safety pin through the elastic's casing, the pocket that you sewed. When it's all the way through, remove the safety pin. Pull the elastic until it is snug and the cover is gathered the way you want it to be. Cut away the excess elastic if any. Overlap the ends of the elastic by about half an inch, and sew through them on the machine. Sew back and forth a few times to make a strong connection that won't tear free.
Hand stitch the elastic's casing closed, using a hand sewing needle and thread that matches the fabric. Insert one of the free edges into the other, and stitch through both of the layers. Make small stitches close together. Tie off by sewing several stitches in the same place, and cut the thread. Put the slipcover on your cushion.
Square cushions can be slipcovered, too. One simple way to do it is to round the fabric and hand stitch the elastic casing closed on each corner, as you did the one place on the round slipcover.
Fabric type and weight will affect how it gathers and how many times you will need to snip the fabric to get it to lie right. Experiment to see what you like.
If the elastic isn't long enough, you can extend it by sewing another piece to it.
Different fabrics will require different threads and needles. Check your sewing machine's manual for information.