Tapping DOME lights will require running more wiring than necessary, unless you want to use DOME lights being "on" as the switch for your rock lights.
The process itself is dead simple, but when it comes to electrical, it's a good idea to find a buddy that knows what he's doing, and have him show you how to do it. A cut of the wrong wire can cause serious issues with the vehicle, so electrical is not something I suggest people play guessing games with or learn no.
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That said, you'll generally:
0. Disconnect the negative terminal on the battery, and ensure the terminal and wire cannot accidentally come in contact, by placing a folded towel or something similar in between them.
1. Install a switch somewhere.
2. Install a regular 12v automotive relay somewhere.
3. Run a (fused!) wire from the battery's positive terminal to the appropriate pin on the relay.
4. Run a wire from the switch to the appropriate pin on the relay to turn the relay on/off.
5. Run a wire from the relay's appropriate pin to your rock lights. When the switch in step #4 is turned on, the relay "passes on" the power from wire in step #3 to the accessory in step #5.
Or if you want to use the DOME light switch as the switch for rock lights:
3. Find the DOME wire that gets 12v power when the DOME lights are on, but does not get 12v power when the DOME lights are off.
4. Tap the wire from #3, install an inline fuse, and run the wire to the appropriate pin on the relay to turn the relay on/off.
5. Same as above.
