If you haven't already experienced it, you probably will - your blood pressure rising as you watch your (or someone else's) capital bounce off of a structure. It's not fun, but with a few simple steps, it can be avoided.
This article will also guide you through general advice to enhance your safety with cynosural mechanics. It will not cover lighting a cyno in a combat situation.
You will need a corvette, liquid ozone (250-450 units depending on your ship class and skills), and a Cynosural Field Generator fitted to your ship.
Selecting the drop point
The safest possible place to light is in tether range of a citadel large enough to allow the incoming capital docking access. If there is no appropriately sized citadel nearby, it is safer to light inside the docking ring of an NPC station. This is because of the tether delay introduced in early YC120 - that capital is a sitting duck if it can't dock.
Okay, so you've identified a citadel large enough to allow instant docking access (and, thus, safety) for the incoming capital. Well done, pilot. But your job's not yet done - and neither is your responsibility for that capital's safety.
The trick is to open your tactical overlay, the button for which is located to the left of your shield HUD. Zoom out, and mouse over the cyno module. A red orb will light up in a 5K radius around your ship. That's the degree of variance in the insertion point for the capital. Furthermore, most are quite large in diameter - even if the orb is free from obstructions, an extra few kilometers of clearance is needed.
So a good rule of thumb is 10K clearance (slightly more for a titan) from any structures, other ships, and, of course, the edge of the docking/tethering ring. Once you've established that, you're all set to light.
Good luck out there, pilot.