I used to do this all the time. any more it is cheaper to buy ready-made slipcovers from catalogs–Penney’s, Brylane Home, spiegel–as the fabric and accessories are getting more expensive, and you have to waste so much to cut it to fit. got a brown suede-look slipcover for an old vinyl sofa that had dried up in the Texas sun in our family room, $35 on sale from Brylane, which looks great, I bought pillows from the dollar store and made my own bright shams and covers, it looks like something I saw in the Pottery barn catalog for $2000, whole thing was less than $50. Don’t even try to match a plaid or pattern for your first attempt, try a solid color. I found double-knits, heaviest guage I could find, worked best when I had kids because the fabric stretches when kids jump around on furniture, and that fabric has give, and easier to tuck back in and make it neat. I did reupholster a recliner after our dog ate the foot rest and front of the chair (yes, ate, not chewed), but nearly had a nervous breakdown.
upholstery fabric, lining and batting is expensive, so don’t make the investment unless the frame is in good shape, and the piece will hold up under at least 5 years of use. A cheap chair on sale is probably cheaper than reupholstering, so slipcover first. If the piece is okay to sit on but the seat sags, the slipcover places sell boards to put under the seat to make it firmer, which will give you some more mileage.