The easiest baby costume ever you will ever craft is the following Stick Figure costume I created. All you need is two items: a white pj footie onesie and black washi tape (or black ribbon)
I only had one plain white footie onesie and it was a NB size, so I found one at Babies R Us from Carter’s. If you have a white onesie but have markings or a cute print, then simple turn them over or wear them in the reverse side.
Step One: Attach black washi tape or Hot Glue some black ribbon to the onesie.