It sounds like you're better off sewing from raw fabric, then you can make the duckbill flap collar characteristic of onesies, plus the hem won't end up as sloppy as the pictures in that tutorial. Do you know how to work a machine? I don't mean to patronize if you're not a novice, but I'll spell it out for any other anons too.
If that tutorial isn't clear, it looks like you'll need to understand how to work your machine, and pattern drafting. There's loads of tutorials on youtube about sewing, but those are the two key things you need to know to get you started on any project, along with buttons or snaps, depending on your preference.
After you understand those, the next is copying the pattern. I recommend buying a baby onesie, cutting the seams and laying out the cuts (the pattern pieces). Do the same with an old T-shirt that fits you, and add the brief shape on the hem and the duckbill collar on the new pattern. The baby onesie is to give you an idea of what it looks like, but keep in mind that baby proportions arent going to be the same as your own.
It takes some work to learn, but you'll never have to buy adult baby clothes again. Also a fun thing to try later down the road. I use the duckbill collar for T-shirts, which I wear as street clothes. Kind of a distinguishing element to have in your ensemble.