Tension - 15 stitches by 18 rows is equal to 10cm (4") in stocking stitch.
Finished size of bag is 40cm (16") wide by 32cm (12 3/4") deep.
Abbreviations used -
sts = stitches
k = knit
p = purl
st st = stocking stitch
inc = increase
rep = repeat
cont = continue
beg = beginning
gs = garter stitch
tog = together
Method - With your yarn double up cast on 40 sts using the 6mm.
k2 row in st st (1 row knit 1 row purl)
(1) k3 sts, inc 1 st in next st, k to end of row
(2) p3sts, inc 1 st in st. p to end of row
Repeat both rows above until you have 52 sts ending on a purl row.
Next row: * k6, inc 1 st in next st, * repeat from * to * to last 3 sts k1 inc in next st k1, (you should now have 60 stiches on your needles).
Continue in st st (starting with a p row) until work measure 27.5cm (11") from beg ending with p row. From here change to garter st (every row knit).
(1) k3,k2tog, knit to end of row. Continue as row (1) dec beg of every row until 48 sts remain.
Knit the second side to match.
Making up - Sew together at both sides and bottom by oversewing.
If you wish to line then cut lining based on shape of bag and sew around edges then hand sew into place inside the bag.
Sew on handles at centre of both sides at the top opening