The diameter of random Schreier graphs

We give a combinatorial proof of the following theorem. Let $G$ be any finite group acting transitively on a set of cardinality $n$. If $S \subseteq G$ is a random set of size $k$, with $k \geq (\log n)^{1+\varepsilon}$ for some $\varepsilon >0$, then the diameter of the corresponding Schreier graph is $O(\log_k n)$ with high probability. Except for the implicit constant, this result is the best possible.