It kind of depends on the type of chairs you have, Mike, but I have a tutorial for stapled patio chair cushions, if this is the type you have: https://lorabloomquist.com/refreshing-patio-chairs-stapled-cushion-how-to/
Yes it would depend on the type of cushion - wheter is has a zipper, or your going to have to open the back seam. Then slide the foam out and replace with a firm/med. firm foam slightly thicker than whats in there now. If you are sewing a new box cushion... here is a video... https://www.youtube.com/watch?v=FFukCzW2zDU
If the cushion is stapled onto a wood base, pull the staples out, and use an old piece of the fabric to cut a new one, cutting at least 1/2" larger. Cut a new piece of foam to fit the wood base with 1" extra around. Then center your fablic on top, flip it over and staple around, checking your pulling snugly. I like to do the middle of all 4 edges, to make sure the pattern is straight and there are no wrinkles, the I finish each side. I use 1/2" staple. Then cut a piece of poster board or black cotton to cover the underneath and staple on.
I would use Sunbrella fabric or some exterior upholstry fabric thats been treated to hold up to sun/UV.. I get mine on www.fabric.com and its 54" wide so you can usually get 2 cushions for a table type seat.... https://www.fabric.com/find?searchText=sunbrella+outdoor+fabric
I've bought them from here if you don't want to DIY. Depends on your furniture, needs but the replacements were better quality than the originals. https://www.gardenwinds.com/
This time of year is a good time to check out your local Goodwill and Salvation Army. People got new outdoor furniture and take their old stuff to those places. We just picked up a long cushion for our cinder block bench in the front yard. Paid less then $5 for it and it is in perfect condition. Plus our local Salvation Army is having a 25% off store wide sale right now too.