Start with $N_{0}=G$. Once you have $N_{i}$, look to see if there are any nontrivial proper subgroup of $N_{i}$ that is also a normal subgroup of $G$. If there is, then let that be $N_{i+1}$ and repeat. If not, then you are done, and $N_{i}$ is what you get. The process must halt, as $|G|$ is finite.
Really, I do not see how being solvable is relevant at all.