Start with the biggest, Cosby-est sweater you can find at the thrift store.
Sweaters that look good as is - leave them for someone who needs it.
Knit sweaters with lots of holes - I learned through trial and error that nothing will stop sweaters like these from deteriorating in the wash.
Totally Go For -
Horribly ugly sweaters, so bad they're good
Sweaters that have sleeves as long as your leg
Put the sweater on upside down and inside out, your legs in the sleeves. Belt or otherwise secure the bottom of the sweater at your ribs.
In this totally awkward position, baste the sleeves of the sweater so that they are as tight/loose as you desire. Baste shut the neck hole. If you can, chalk along where you basted to make it more obvious when you go to sew.
Cut off the bottom of the sweater, leaving enough room to make a waistband later.
Take your pants to the machine and follow the line you made basting. If you feel like you are sewing blind (as I felt) lean towards sewing too big as its a serious pain to stitch pick in this situation. Try the pants back on and make adjustments as needed, pinching and basting and sewing until you have a reasonable fit.
The crotch probably looks weird, I know mine did. You're going to put a dart in to make the crotch area look reasonable. I put my dart in the front because it made the pattern pull up into a point, and I liked that. Also a dart in the back seemed way more likely to split.
To make the dart I just tried on the pants, pulled them into shape and basted.
Create the waistband by folding the top of the pants down the width of the elastic and sewing, leaving a gap at the end. Cut a piece of elastic to size and thread through the casing, sew the ends together and close the hole. Ta da!
They are surprisingly comfortable and pants-looking. I would not bank on getting a whole lot of compliments, especially when matching sweater pants with giant sweater to make a sweaterbeast.