If you have old unused fabrics or old used fabrics (bed linen, curtains, dresses, shirts etc.) you can reuse them again in craft making. Old clothes have always been used in patchwork but you can use them also in weaving and crocheting; why not in knitting too.
Cut the fabrics in this way. Pay attention on the turn ups. You can use the scissors but I usually use the scissors only to cut the start and then tear the rest. Depending on the fabric it may be very easy to tear it. Usually the easiest direction is according to the weft but sometimes it works better to the warp direction. I tear it up always when possible to work fast but also to get the weft straight and even.
How wide you should make the rag weft to be? It depends on it how thick the fabric is, what you are making and by which method. 1½-2 cm is max for the crocheting and knitting so that it still is easy to work with without stressing too much your hands. (Take big enough hook or needles.) For weaving you can use also thicker if you like. Notice that if you tear the rag it vanishes few millimeters by both edges.
Wind up the rag weft turn out to be a ball. If you are using flexible material be aware of not to stretch it. Wind up always over your four fingers which hold the ball, then loose the hand, change direction / point a bit and wind up again. The ball should not be too tight so that it can hold in the material without damages (stretching & getting wrinkled too much).
For rag weft you can use also old VHS and Cassette tapes. There is a lot in each so open one at a time to avoid a huge mess with them. You can use VHS and Cassette tapes straight from the reel that's inside. If you remember or happen to know 😊 Cassette Tape is much thinner than VHS tape so it suits better for smaller hooks.
Old fabrics and tapes you can ask always from friends, buy at second hand and charity shops. Make sure everything is clean before using. It is very eco-friendly craft making but also excellent method if you need to keep up the budget. Not to mention to save those old pretty / emotionally valuable pieces of clothes your heart doesn't allow you to throw away but brains keep on nagging you can only hide them in your wardrobe or attic!
What to make of rag? Rugs and baskets are good to start to try it out.