First off, make sure that you have various sizes of shirts and onesies. A newborn may probably be only able to use a onesie once or twice before it's outgrown. Have a good mix of shirts and onesies in a smaller quantity of newborn sizes and a greater number of larger sizes up to 18 or 24 months of age. That way, the mommy-to-be will be able to space out the shirts and onesies and not end up with a mostly unused pile of tiny clothes.
While most baby shirts and onesies are, no one says they have to be white. You can definitely purchase colored shirts and onesies and these can be the base of really beautifully decorated works of art.
Before anything is decorated, they do need to be washed. Choose an unscented, mild detergent or laundry soap to do this so that there is less chance of irritations and allergic reactions when the newborn wears your creations. Allow them to sun dry or dry them on low in the dryer. Cut out a stabilizing board for each baby shirt or onesie using a thick cardboard that is exactly as wide as the baby clothing so it won't move and stain.
Prepare a good mix of fabric paints and pens to decorate the clothing. These don't need to be the only decorating materials, however. Provide some iron-on paper and a digital camera and printer for those guests who are more comfortable behind the camera. You could also have some pre-cut fabric forms available that are already stuck to fusible web. All the guests need to do here is to choose forms to create a design, pick up a hot iron, and voila! Instant decorated shirt!
There are also other iron-on designs available, from patterns to letters that can be used to create very original artwork. The less intimidated guests will probably use a combination of materials and methods to make their shirt or onesie decorating one of a kind.
To get everyone's creative juices pumping, you can provide them with a starting point with a list of favorite slogans or sayings. Something like “Hot Mama = Pretty Baby” or “I saw, I threw a tantrum, I got” could be enough to generate a creative design or offbeat idea.
Have your decorating corner well organized. Setting your supplies in their own corrals is a great way for everyone to see what is available to use. Have two or three irons and ironing boards ready for use, as well as a couple of strong hairdryers.
Finally, don't provide any beads or rhinestones for decorating
since the baby can pull these off and swallow them. They are definite
choking hazards and you don't want to be responsible for any avoidable
accidents. Use the puff paints and glitter pens instead.