@twinklesamir A 4/5t sized jacket. Maybe you can find some used if you don't want to invest in one because you don't know which size your kid will be in a couple of years.
You put the baby in the carrier. Then you'll open the jacket and flip it upside down. Baby's legs go into the arms openings. You could tie the ends of the arms of the jacket with rubber bands to prevent wind getting inside.
Then you'll just pull the rest of the jacket over the panel, you can either use safety pins to pin it at the straps or clothes pins or just tuck it under the straps of the carrier. Hope this makes sense. So where the kid's butt would be if a preschooler would wear the jacket, that part covers now the top rail of the carrier. Baby's butt is covered by the neck of the jacket.
Extra tip: Take a large belt or something to get around you because otherwise your jacket/coat will be open and it'll get cold.
You could also ask in your family or ask your friends if someone can spare a jacket that's two sizes of the size you'd normally wear. That should be enough to close it around both of you. I used my step dad's skiing jacket.
Bundling up in layers is not recommended. First thing is that the baby will no longer be ergonomic. Also wool is the only thing that would keep the baby warm since it's not poofy. Winter clothing works with air between layers. When babywearing those layers get compressed so it won't be that warm.
Then baby could overheat (especially if your going in and out of stores). Best thing is to share body heat. Your temperature will help to regulate your kid's temperature.
Third thing: you don't have to stay in the cold when the baby is asleep. Terrible mistake I made back then. I had trouble to get the kid to nap but I also needed to go somewhere. So I've bundled her up good (fleece and wool). When I was done she was finally asleep and I went home because the wind was a bit cold. I came home, stood in front of the door and realized that she would wake up if I took her out of the carrier. I literally just got her to fall asleep, the night was shitty so I didn't want to risk anything. So I went downstairs again, tried to find a wind-free spot and just stood there because she was warm enough. Next times I put her in the wrap in normal clothes, put either a cover on or or big jacket. Whenever I came home, I took off my clothes and she didn't overheat and I didn't have to take her out.
