eta: the stickers were great b/c I only had about 4 white onesies that I used for the whole year since he fit in the same size for several months. and since I could just take the sticker off and put the next one on the next month that made it easy. I would not want to use iron on b/c then you would have to buy 12 onesies, KWIM?
Re: DIY Iron On Monthly Decals
I can't help with iron-on, I've never seen those myself
For DS1 I used the sticker kind, from this website
https://pickysticky.com/
eta: the stickers were great b/c I only had about 4 white onesies that I used for the whole year since he fit in the same size for several months. and since I could just take the sticker off and put the next one on the next month that made it easy. I would not want to use iron on b/c then you would have to buy 12 onesies, KWIM?