Almost exactly what Heated/Fuquan said.....

You add whatever is missing. Since you can sample all sorts of things, you are going to have to add in elements. For example, let's say that you sampled a piano solo. At minimum, you are going to have to add in a rhythm section. Or maybe you sampled a clean bass line - then you are going to want to add in some drums, and likely a higher pitched melody. Add in whatever it takes to flush it out and build a song.