They are simple, quick and ususally well received. I love to make little personalized cards and gifts. I started doing these a few years ago. Here are two I just did this weekend for a friend.
I stamped "Brooke" using the Skinny Dip alpha with Ranger's Black Archival Ink. I like the impression the dye ink leaves over a craft ink like Fabrico (now called Versacraft). The dye ink gives a crisper image. I used one of the stamps from the Ornamentals set as the "wings" on the heart. The heart is from the Doodle Heart Set. They are also stamped with the same Archival Ink. I colored the name and heart with a fabric marker I picked up at Michael's (I'll have to check on the brand).
Here is another using one of the Doodle Flowers and the same alpha as the above. The grey is a Fabrico marker. I also tried a different fabric paint pen I picked up at Michael's. I think it is a Marvy one. It is the lighter pink on the flower. I do prefer the markers over the paint pen. You just have to watch the bleeding of the markers once you apply it to the fabric. I think next time I will work with a heat gun and after coloring a section give it a little blast.
These are really simple and quick to do. Just make sure you wash the onsies (t-shirt, etc.) first. Then put a small cutting mat or something hard inside to stamp on. This will also prevent anything from bleeding through to the back side. Once you are done stamping and coloring you need to heat set the inks. I usually hit it with a heat gun and then toss it in the dryer for a bit. You can iron it too.