If you have not already done so, first open the nitrogen tube at the splice that is located about a foot from the etalon, to ensure that dust is not expelled into the etalon when you turn on the flow of gas. Then open the left valve on the regulator to start the flow of nitrogen. Turn the valve until the left regulator gauge reads a few PSI; then use the needle valve in the nitrogen hose to reduce the flow to a more suitable level. (The regulator valve is probably not well-suited for maintaining a flow as low as is required by the Fabry-Perot. However, do not open the regulator valve too far, as the pressure may stress the seal where the hose attaches to the regulator.)
While adjusting the needle valve, try to detect the nitrogen flow from the end of the tube on your lips. The flow is about right when you can definitely feel it on your lips, but only just barely. If the bottle pressure drops by more than about 200 PSI over a single night, then the flow is probably set too high, and you will be in danger of using up the nitrogen before the run is over. (If this happens, the Palomar staff should have additional dry nitrogen on hand, but don't count on it!)
When the flow has been properly adjusted, connect the tube to the etalon. You may want to check the nitrogen flow and the tank pressure periodically during your run, to make sure the nitrogen is being delivered at an appropriate rate.