I did this a couple of years ago and have been happy with the results. My neighbor, a thrifter-extraordinnaire, had found two dirty but sturdy white plastic chairs with old icky cushions on a street somewhere waiting to be picked up with the garbage. She rescued them and gave them to me. I scrubbed and scrubbed both the chairs and the cushions but was never able to get them clean-looking. So I decided to make them over.
I then took the cushions and decided to cover them with red outdoor fabric. I just traced around the pillow with a pencil right onto the fabric, making the outline about an inch or so bigger all around than my pillow. I sewed right sides together, flipped it inside-out, and then sewed on Velcro as a fastener. This allows me to wash the cushion covers regularly...they always seem to need it, especially because our cat likes to curl up on the chairs.
The old cushion still lurks inside....
but I can velcro it shut!
I admit that the red and black color scheme is not at all my thing, but it seemed to work because we have a grey house, black front door, and I value simplicity. This looks nice, especially in autumn when I load up the porch with mums and pumpkins. They also look great with the Christmas decorations and blend nicely with anything we might do for summertime.
And these chairs are *very* comfortable. I love to sit out here and read!