I used worsted weight yarn and 4mm crochet hook, but really - gauge is not important for this project. Just make sure the stitches are tight enough so that the stuffing doesn't show through.You can change colors at any point to make an interesting pattern.
You can learn basic stitches here.
abbreviations:
ch - chain
sc - single crochet
st - stitch
idec - invisible decrease. Instructions for this can be found here.
Instructions:
The ball is crocheted in the round, to not turn or join. Place marker in the first stitch of the round and move up as you go along, it makes keeping count easier.
ch 2.
row 1: 6 sc in second ch from the hook.
row 2: 2 sc in each st (12).
row 3: [2 sc in st, sc in next st] 6 times (18)
row 4: [2 sc in st, sc in next 2 st] 6 times (24)
row 5: [2 sc in st, sc in next 3 st] 6 times (30)
row 6-13: sc in all st.
row 14: [sc in next 3 st, idec in next 2 st] 6 times (24).
row 15: [sc in next 2 st, idec in next 2 st] 6 times (18).
Stuff the piece with fiber fill.
row 15: [sc in next 1 st, idec in next 2 st] 6 times (12).
row 15: [idec in next 2 st] 6 times (6).
row 16: [idec in next 2 st] 3 times (3).
Fasten off. Weave in the end to close the hole.
