You can buy the Clorox Bleach Pen
at grocery and other stores. Probably most places that would sell bleach since it's marketed for things like grout or small stains. I got mine at Target. It's in gel form with one end being a pointy tip the other is a brushy tip.
I used a 'template' for the stars, but did the flowers free hand then filled them in. I used only the pointy tip. If you don't want bleed through don't lay it on too thick and it'll only bleach the top strands.
When the design was white I held them flat under running water to rinse the bleach out of the design areas. I then used Woolite and washed them out well. The star one I then tossed in the washer with some other dark clothes like jeans then threw it in the dryer. The flower one was just squeezed out and tossed in the dryer with some dark towels. I didn't have any problems with the other laundry or areas on the bibs probably because I rinsed them before hand.
Note: the yarn is supposed to fade like jeans so always wash with dark or similar colors. I worried a bit about the navy blue one so washed it by hand and I was not disappointed. The water was really, really blue! Just a heads up to be careful.
In addition, since the yarn naturally fades the contrast with the bleach areas will probably become less contrasty, but that's okay IMO. It'll just be well loved.