Most families place their Milestone Card next to their baby and snap a photo!
You can also tape the card to the outside of an isolette if a baby is too delicate to be moved or disturbed.
Some families don't like posting pictures of their baby in the NICU - it can seem more private or delicate than they want for social medial. So, you can take photos cards corresponding to the milestones your baby has achieved and share that on social media to keep your friends & family updated without having to share pictures of your baby.
If you want, you can use a pen to write the date on the card to remember exactly when each milestone was reached.
If you're fortunate enough to have a professional photographer do a photo shoot in the NICU, you'll definitely want to use your cards. But if you are stuck with just using your phone camera (which is true for nearly everyone), check out this great guest post from the incredibly talented Jessica Strom: How to Photograph your Baby in the NICU with your phone.