In MA, the only legal way [1] to travel by bike with a baby is with a trailer: they're still too floppy for a child bike seat and the law doesn't consider options involving the box of a cargo bike. Since they're so small, you can use a car seat to make sure they're properly supported and protected.
We have a Burley Bee (which I still like a lot) and today we went for a short ride with Nora:
I used three cords, attaching to the trailer at six points: two points at the top back, two at the bottom back, and two at the bottom front. Here's a diagram, with the frame in solid blue and the cords in dashed red:
It was a bit annoying to get it all set, but once it was connected and tight the seat was very secure. I did this all before putting Nora in: there's plenty of room to slide the baby down under the cords and buckle them in as usual, which also meant that I didn't have to redo the attachment when it was time for the return trip.
I biked on the slow side (and I'm already not fast), so as not to jostle too much and give plenty of time to avoid anything dangerous. We didn't use a helmet: it's not recommended for kids this young, and you need to be protecting them well enough that their head would not hit anyway.
[1] In MA
the relevant regulations seem to be (I.XIV.85.11.B.2):
The operator shall ride only upon or astride a permanent and regular seat attached to the bicycle; a passenger shall ride only upon or astride a permanent and regular seat attached to the bicycle or to a trailer towed by the bicycle.
The operator shall not transport another person between the ages of one to four years, or weighing forty pounds or less, on a bicycle, except in a ''baby seat'', so-called, attached to the bicycle, in which such other person shall be able to sit upright; provided, however, that such seat is equipped with a harness to hold such other person securely in the seat and that protection is provided against the feet or hands of such person hitting the spokes of the wheel of the bicycle; or upon or astride a seat of a tandem bicycle equipped so that the other person can comfortably reach the handlebars and pedals. The operator shall not transport any person under the age of one year on said bicycle.
Any person 16 years of age or younger operating a bicycle or being carried as a passenger on a bicycle on a public way, bicycle path or on any other public right-of-way shall wear a helmet. Said helmet shall fit the person's head, shall be secured to the person's head by straps while the bicycle is being operated, and shall meet the standards for helmets established by the United States Consumer Product Safety Commission. These requirements shall not apply to a passenger if the passenger is in an enclosed trailer or other device which adequately holds the passenger in place and protects the passenger's head from impact in an accident.