For these mods to be of any use you'll need the original BSJ pattern. I don't want to breech anyone's copywrite by posting the pattern, but hopefully these instructions will make some sense. If not, let me know please.
To make a newborn size BSJ using 100g (or a bit less than 230m or 250y) worsted weight wool and size 4.5mm needles (I haven't actually worked out my gauge but I'll have a look later)
Cast On 128 stitches
Mark stitches 29 and 100
After 8 rows (4 ridges) increase 7 across each end
At 70st work 3 even
At 86st work 8 evenly across middle
At 114st shape neck by bind off 4 at each end
At 118st work on centre 66 for 16 rows (8 ridges)
Mark stitches 34 and 100
4 rows then make buttonholes then 4 more rows
Then you should end up with something cute like this:
