5.5 mm crochet hook
~100 g 8 ply yellow wool
~100 g 8 ply black wool
In yellow wool:
r1 ch4, 24 foundation dc, join with a sc in 3rd ch (24 dc)
r2 sc over the top of the joining sc, ch2, dc in the spaces between dc around, join with a sc in 2nd ch
r3 change colour by incorporating the black thread in the last loop of the joining sc (don't cut yellow), sc over top of join, ch2, dc in spaces, join with a sc
r4 repeat r2
Repeat rows 3 and 4 alternating yellow and black until have 12 stripes (6 yellow, 6 black). In order to keep the seam straight, it'll be necessary to join the occasional rows with a sl st, sl st in the space between the ch2 and first dc, sc over the top of the sl st. This puts the join on the opposite side of the ch2, which makes the join line reasonably straight. I did this about every 5th row.
r25 change to yellow. Sc over top of join, dc around, add an extra dc into same space as first sc, ch2. (25 sc)
r26 repeat r2
r27 repeat r3
r28 sc over top of join, dc around, add an extra dc into sc space (26 sc)
Repeat rows 27 and 28 four more times until there are 30 stitches. Add 5 more stripes, ending on yellow.
You can easily adapt this pattern to suit different sized arms by using a different number of stitches and making the increases happen earlier or later. Also, I have long arms because I'm 175 cm (5' 9"), and the sleeves fit comfortably from the base of my thumb to my elbow. If you have shorter arms, you'll probably be wanting to take out some of the stripes. It's all easy to do if you measure it as you go and just start to increase as your arm gets wider.