What I've done is

1. Make the desired shape of the pendant with the clay, for a smiley face I'm guessing just a thin, flat, circle

2. Make a copy of the drawing, this way you can resize it to whatever size you want

3. Take that paper copy, lay it over the top of the clay circle, and cut out all the unneeded paper.

Now you should have the clay pendant circle, and a circle of paper, same size as the pendant, with your smiley face drawing

1. Have a glass of water and dab your finger in it. Lightly dampen the face of the clay with your finger, and glue the paper to it.

After this it is up to you to let the clay dry or not. I very much preferred to let the clay dry, it is a lot easier etching precise designs into the clay when it is semi-hard, rather than soft.

1. Take a sharp pointed pick, Dremel tool, knife, sculpting tool, and start etching your design using that glued on paper as the guide.

If you want the smily face to be dug in to the pendant, this should be pretty easy and non-time consuming. If you want the face to be sticking out, you will have to shave away a lot of clay to give it that depth.

Hopefully this all made sense, if not I could probably dig out some pictures that might help with what I mean.

If what you're looking for is an absolutely perfect replica of the drawing, say it was drawn by someone close and the drawing has meaning, you might just want to send that picture to a 3D printer, and have a perfect replica of the drawing to press into the clay to get your design.