There are many ways to harvest and prepare your yarn for knitting. These are my methods. For more information and other harvesting/cleaning/knitting recommendations, visit the TBSP Resources page.
Tape measure &/or ruler
Ball winder (optional)
Brown paper bag, large pot, or bowl
The first place I search for sweaters to better is in my own closet. If I don't have anything worthy of upcycling, I make a trip to my local thrift shop. When you're browsing for your contenders, the four things to remember are: 1) condition, 2) fiber, 3) seams, and 4) weight.
1) Condition. That means minimal to no pilling or broken threads.
2) Fiber. Everyone's preferences are different. I'm very partial to natural materials like wools and plant fibers but buy what feels good to you.
3) Seams (oh so important). Commercially produced goods are rarely knit in one piece so check the seams to make sure they are easy to take apart. The best seams are ones that are crocheted and can be removed with a quick tug.
4) Weight. Get that ruler out, it's time to measure the gauge! I like to use Ravelry's Standard Yarn Weights Guide (stitches per 4 inches).
If the sweater satisfies in these four areas, then you've got one that's ready for harvesting!
Info coming soon.
MEASURING & SKEIN-MAKING
Info coming soon.
Once your yarn is harvested and wound into a skein, clean it by submerging in a bucket of very hot water (lukewarm for fibers like wool or cashmere or alpaca--there might be some shrinkage) with a bit of shampoo or wool wash. Submerge for roughly an hour (I sometimes put a plate on top of the yarn to keep it from floating to the top). Then squeeze out the skein (gently so as not to accidentally felt it), rinse with more hot water, and squeeze out again (skip rinse step if using wool wash). Next, pop skeins in the washer and put on the spin cycle. Remove from washer and hang up to dry. If that step makes you anxious (or you don't have a washer), just lay your skeins out on a towel, roll it up, and squeeze. Remove from the towel and hang up to dry. For more info on cleaning, check out Dawns’ instructions here.
Like you would with any skein you purchase, you need to wind your yarn. You can use a ball winder or do it yourself by hand winding a center-pull cake (here’s a video how-to).
PATTERNS & KNITTING!
If you're already an established knitter then the following will be old hat. If you're not, here's my recommendation for finding patterns. Join Ravelry and add patterns to your virtual library. (I add anything I like and generally purchase the pattern when I’m ready to begin knitting. You can get enough info about gauge and the amount of yarn you need from the pattern page on Ravelry without purchasing the pattern.)
To find a pattern that’s a good match for the yarn you’ve harvested, click the “search patterns” button (under the “add items to my library” button) and sort your personal library by yarn weight. For more inspiration, you can also search for patterns by weight in the general Ravelry library. I explain this with images in the journal post "Finding the Right Pattern."
Always always set up a swatch to establish the correct gauge!
Have fun and enjoy your better sweater project!