I'm a big fan of Milliput. I bought my first package of it on a trip to England in 1978, and I've been using it ever since - for all sorts of things, from making the "carvings" on a sailing ship model to repairing old plaster figurines.
I can offer a few tips.
1. It's available in several colors and textures. I don't think there's really much to choose between the textures; "standard," to me, works just about the same as "fine." The color doesn't matter much either - assuming you're going to paint the finished model. But the choice of color does make a difference in terms of how the finished paint looks. I used the terra cotta kind to make some parts for my Revell Viking ship, because it's just about the color of the plastic. If I'd used white or grey, I would have needed several more coats of paint (I brush paint models like that) to make it blend in with the surrounding areas.
2. When you mix the two parts together, make sure you really mix them. It takes five or ten minutes. If you see streaks of the two colors, you need to keep mixing. If you don't, the stuff may not dry right.
3. One of Milliput's big advantages (if you look at it that way) is that it takes a long time to dry. Usually two to three hours - though the time will vary depending on the thickness of the application. When it's just been mixed, it handles about like modeling clay. When it's almost but not quite hard, it works like hard wax. In that condition the only limit on its detailing ability is the modeler's skill.
4. The instructions tell you that it will be easier to smooth if you put a little water on it. That's true, but use the water sparingly. The only time I've had a problem with Milliput crumbling was when I had to repair the cast plaster details on an old, cheap picture frame some friends wanted restored. I mixed some water into the Milliput, thinking it would stick to the underlying plaster better that way. Mistake. As soon as it dried it started chipping and crumbling. Don't apply any water until the Milliput is shaped pretty much the way you want it. Then dampen youre finger and touch a little water to the surface.
5. One of Milliput's most remarkable characteristics is its ability to hold its shape when rolled out into thin sheets. I haven't tried it myself, but some 54mm figure modelers make flags out of Milliput.
6. I know of at least two mail order firms that sell it: Squadron and Micro-Mark. I've also bought it at, of all places, Hobby Lobby.
Hope that helps a bit. I highly recommend Milliput. Good luck.