I cut out the shape of a tie with some scrap fabric, and this iron on 'stuff.' You peel one side and iron it to the fabric, then peel off the second and iron it onto the shirt/onsie. You don't have to sew it on, it supposedly sticks, or can be pressed back down with an iron if the ends curl, but I usually iron it on for looks.
I also sewed some 'stripes' along the tie before place it on the onsie for looks. It would be cute with red thread as well, but that requires changing out the bobbin. Some things are just too hard. Or is it...sometimes, I'm just too lazy?
I don't actually expect/or want an answer to that last one! I'll use my imagination to guess the response of friends.