The general idea is to create the greyscale heightmap of a crater-like structure. You can go about that in a number of ways. I've attached one made from scratch in photoshop using white circles that were subsequently blurred to create a gradient-like effect. Below that, a photoshop file which is ready to be turned into a custom brush, making it easy to apply the effect to a larger image multiple times. To make the crater bumpmap for entire texture, make an image the same size as the texture, fill it with black, and use the brush to place craters.
Then, to make the craters appear on your texture, save the new bumpmap as a .psd, use the texturizer filter of photoshop, load the new bumpmap into the filter, and adjust lighting and how much relief effect you want. The size parameter should be 100%.
At the bottom, I've attached a simple example of having used the crater brush on a simple noise map.
This is not meant to be the final word in crater making; it's just one way. You should experiment
EDIT: In case you meant how to make the craters as part of the 3D mesh, that is generally not advisable - it requires a lot of polygons to look good, and polygon count should be kept relatively low for realtime 3D applications such as US2.