I just improvised a pattern that would work well, its in the round so you only need to use the knit stitch
you will need a circular needle (like a 29" needle in the size needed for your yarn... probably somewhere between a size 7 and an 11)
Cast on 200 stitches, Place marker (work in round), knit around for about 30 inches
Bind off 80 stitches, knit 20 stitches, place 20 stitches just worked on stitch holder, Bind off 80 more stitches (there should be 20 stitches left)
Work these 20 stitches in Garter stitch (knit every row) for about 48 inches then join these stitches with the stitches on the stitch holder using kitchener stitch (grafting),
secure bottom of bag (I suggest single crochet, but you could also use whipstitch or backstitch)
AND FELT
The final measurements of the bag should be about 18 inches wide, 20 inches tall, with a 36 inch handle (just right for a shoulder bag, however sometimes the handle doesnt felt well in the washing machine because it gets tangled in itself, so this measurement may vary)