Worsted weight yarn, preferably something easily washable (I used Lily Sugar and Cream cotton)
Size 5mm crochet hook
Button of your choice
SlSt- slip stitch
SC- single crochet
DC- double crochet
Row 1- Ch 4, and SlSt into beginning chain to create a loop. Ch 2, (counts as a DC), and then make 11 DC in the loop, join to the top of the CH 2. (12 DC total)
Row 2- Ch 2, make a DC in same stitch as the CH 2 just made, then make 2 DC in each stitch around, join to top of chain 2.
Row 3- Ch 2, make a DC in the same stitch as the CH 2 just made, then (1 DC in the next chain, 2 DC in the next), repeat the parenthesis all the way around, join.
Row 4- Ch 1, make 1 SC in front loops only in each stitch all the way around, join.
Row 5- Ch 1, (make 1 SC and 1 DC both in the same next stitch, skip next stitch), repeat parenthesis all the way around until 4 or 5 stitches are left unworked. 1 SC in the next stitch. Chain 1, TURN.
Row 6- Make 1 SC, and 1 DC in the stitch above the DC from the previous row. Repeat all the way around until 1 stitch is left undone. Make 1 SC in that last stitch. Chain 1, TURN.
Repeat row 6 until the mug cozy is the desired height of your cup.
When finished with last row, Chain about 20, and then slip stitch into the beginning stitch of the chain to create a loop.
Chain 1 to close, cut yarn leaving a tail to weave in, and pull tight the excess yarn through the chain 1 just made, then weave in. (That is my process for finishing off ends, however, you may do so however you wish.)
Sew a button to the cozy on the opposite side of the loop for closure.