Tips on how to make cushions for outdoor chairs?

Use the old cushions as a pattern for your new ones. Take one apart carefully. Then go to the fabric store and find appropriate fabric and lay it out for one cushion so you know how much fabric to buy. Then multiply by how many cushions you need to make.

Google for foam stores in your area. You need upholstery foam versus what they sell in craft and hobby stores. Most foam stores will cut to size for a small fee. Take old foam with you as a pattern.

Take a seat and back cushion apart using a seam ripper being careful to not tear fabric. Use the fabric pieces and the foam pieces as a pattern to cut new foam and material. Sew together fabric with another set of cushions as your guide.

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Use an old cover as a pattern; perhaps use a contrast piping made from bias cut strips to create the cording. OR: use your imagination for a stencil on a casual canvas cover (heck- just clean the cushions and keep the old cover on to hold in more fill if you choose to add it). Do the math for your cushions on your yardage. Check out Home Depot for a canvas drop cloth- they come in a variety of weights. Yeah, they're not weatherproof...BUT you can pop your covers (inside out if you have a painted design) in the wash with bleach and give em a good scrub. They'll get softer with age. Trick: you can use actual house paint to do a stencil design (sponged on) or paint on with acrylic paint. Just know that the design will eventually show a crackle- so think batik look. Or go nautical with lobsters, etc. Toss a rumpled fishnet or rope on a cushion and hit it with a shot of spray paint- you's be surprised what effects you can get ! Maybe even layer two colors like that- or use whatever floats your boat (pun intended) to make a "mask" for your pattern.

In my spare time, I'm a marine upholsterer. If your chairs are outside and exposed to the elements, that explains why the foam needs replacing, it doesn't sound like the original foam was meant for outdoor exposure. As for fabric, you will need to make sure you acquire outdoor grade or marine grade fabric as well.

To make a long story short:

Here's a handy link.

This is a trick I had learned and since used from Trading Spaces. Since you are starting out fresh put the fabric on the right side towards the cushions. Then pin the fabric around the cushion, I drew a line with fabric pencil along the pin lines on both sides of the fabric. Then I sewed along the lines, not sure if you are putting a zipper in or sewing it up. I will try to send a picture of what I am talking about because I may be making it more confusing and harder than what it really is.