Felted wool is a marvelous fabric. It insulates well and it doesn’t fray or unravel when cut. If you were to purchase real wool felt (not the polyester sheets found in craft stores), you can expect to pay around $10-$20 a yard! Skip the fabric stores this round and head to your local thrift store for high-quality material at a fraction of the cost. 
Look for mostly wool sweaters. It doesn’t have to be 100% wool, but it does need to be a majority of wool or another natural animal fiber like angora, cashmere, or merino. I like to buy men’s sweaters; you get more for your money, especially if you can find extra large sizes.
Technically the process of transforming something knit into felt is called fulling and not felting, but I am going to refer to it as “felting” which is more commonly used. I felt wool sweaters easily at home using my washing machine.
Make it!
Put the wool sweaters in an old pillowcase and secure the opening with a rubber band. Trust me, you are going to want to put them in a pillowcase unless you enjoy picking sweater lint out of your washing machine for a long time. Standard laundry rules apply: make sure to keep dark colors separate from the light ones. Set the machine to Large Load, HOT, and High Agitation – basically the settings that would ruin your clothes! Add 2 tablespoons of liquid dish soap to the water, and then throw in the pillowcase with a few pairs of jeans. Turn the machine on and let it fill with water.
Once the machine starts churning, start checking the progress every few minutes. Each sweater will felt at a different pace. Once the sweaters look soft and lose stitch definition, they are felted. Drain the water out of the machine (no need to rinse the sweaters). Remove the sweaters once they reach your desired level of felting and wring them out. You can lay the sweaters out to dry or throw them into the dryer (they will felt more in the dryer so this is my preference).
Now that you have some felted sweaters, you can snip off the sleeves and use them as a cozy to keep warm beverages warm and fingers cool when you are drinking out of a mason jar.
This mason jar is fitted with a Cuppow coffee lid that turns it from canning jar to reusable coffee cup.
If you want to take the project a step further, check out how to make these adorable mug cozies with felted appliques!
After this process, can it be carded for needle felting?
yes!