I've done one before~ The tip is to split it in half, and make sure that the bottoms of each letter touch at the same point. I usually make a file on SAI or PS, with something with layers.
My first layer is usually the layer where i mark exactly where i need the ends to be, so they all match up. Once you have that set, you can start creating the letters according to the markings you made.
The trick to ambigrams is that most people, when reading things, usually read the top half, instead of the bottom half, so it always looks like the actual word no matter what the bottom half is, as long as it has some sort of relevance.