If you already know exactly what you need, that is useful. If you do not, that is normal too. Start with the rifle, the intended use, or the problem you are trying to solve.
Useful details are simple: what you are starting with, how the rifle is used, what bothers you now, and what you want to feel or see improve. That gives Jon enough context to tell you whether the next step is a focused service, a broader evaluation, or simply a conversation before any work is scoped.