I have this adorable crochet pattern for a baby 6-9 months old. I want to be able to make it sizes newborn, 12 mo, 18 mo, and 24 mo sizes. Can anyone help me figure out how to change it? I have only been crocheting for about 2 years and am not all that great at knowing how to increase/decrease. Please help!!

Materials:
Sport weight yarn (Jamie by Lionbrand was used in the models shown)
"J" hook
Size: 20" chest (3-9 months)
Gauge(on yoke): 1 inc and 2 hdc= 1"(measured at the base of the stitches)
*BEG chain does NOT count as first st!
Pattern:
Ch 32
Yoke
1. Sc in second ch from hook and in each ch across. (31 sc). Ch 2, turn
2. Hdc in first 2 sts, 2hdc in next st (inc made). *hdc in next 2 sts. Inc in next. Rep * across to last st. hdc in last st. Ch 2, turn.
3. Hdc in next 2 sts. Inc, *hdc in next 3 sts, inc. Rep * across. hdc in last 2 sts. Ch 2, turn.
4-7. Continue working hdc in each st across with inc in 2nd hdc of inc in row below. Ch 2, turn.
8. Hdc in 1st 5 sts. Inc. Hdc in next 8 sts. Inc. ch 5 (for armhole). Sk 16 sts, hdc in next st, inc in next. hdc in next 8 sts. *Inc. hdc in next 8 sts. Rep * once more. Inc. Ch 5, sk 16 sts. Hdc in next st. Inc. Hdc in next 8 sts. Inc. Hdc in last 4 sts. Ch 3, turn. Do not fasten off.

Body: Beg chain counts as a stitch on these rows
Stitches: V-st[(Dc, ch 1, Dc) in space indicated] Cluster[3dc in same sp]
9. Dc in 1st st. Sk 2 sts. *V-st in next, sk 2 sts. Rep * across to last 2 sts. Dc in last st. Ch 3, turn
10.Dc in 1st st. Cluster in each ch1-sp of v-sts. Dc in beg st. Ch 3 turn
Rep 9 & 10 for body pattern to a total of 16 rows from beginning. Do not fasten off.

Edging
Stitches: Picot[ch 3, sl st into last st made]
Sl st up front in ends of rows, adding 3 evenly spaced button holes (ch 1 or ch 2 instead of sts) between rows 1-8. Sc around neck, adding picots if desired(for lacier look). Sc down other front. Sc across bottom placing picots every 3rd st. Fasten off.

Sleeves
Attatch yarn to underarm. Place between 20-23 sts around. Join. Ch 2, without turning, countinue to work rnds of hdc until legnth of sleeve is desired.(about 10 rows).
11. Hdc-dec in every 3rd st.
12. Sl st around loosely, placing a picot every 3 sts. Fasten off.
Repeat for other sleeve