Using DK so this means following sts for 16” will give a chest to fit 20” so adjusting length of sleeves and body to be 6.75”.

Converted pattern to be seamless.
Sleeves - cast on 34sts, then increased on last rib row to have 2sts less than pattern.
Body - cast on 4sts less than pattern i.e. seam sts omitted.
Sleeves & body joined on same row of cable pattern after 6.75”.
CDD for raglan shaping with the centre st being a sleeve st.
The pattern required decreasing 2sts on each RS raglan edge for a half of it I.e. 4sts at each ‘seam’ so I used a CDD on the alternate ‘WS’ row too.
Neckline sts left on hold for neckband later (not cast off).
As the neckband is folded to inside I joined it with kitchener stitch to give an invisible and stretchy join.

Shown on 8 week old baby.