Sounds simple but it's amazing how many people mess this up. Firstly there are two different ways to place the pickup. Nissan have provided a loop in the wiring loom that drives coil No. 1. You'll find it hanging out of the loom right at the back of the head - near the firewall. Try this one first - just hook the sensor of your timing light through the loop. I've got an old crusty timing light and this loop works fine with it. Other people have had trouble using the loop and prefer to sense directly off of No. 1 plug by making a plug lead extension. Seems the basic timing lights work best with the loop - the "dial-back" lights need to take signal direct from the plug as they are more sensitive and can false trigger on the wrong edge of the signal.
Prerequisites:
Engine must be at normal operating temperature.
Idle speed must be correct. Nissan ECU's use IGN timing to trim idle speed. If idle speed is too low the ECU will add timing, if it's too high then it will pull timing back. In reality timing affects idle speed - and idle speed affects timing. So you sometimes need to go back and forth between the two adjustments to get them both right.
ECU must be seeing "TPS Closed". This is a flag which is set inside the ECU when the throttle is closed. Some engines use a switch on the TPS to provide this - SR20 ECU's simply look for a certain voltage on the TPS (Throttle Position Sensor). Refer to TPS Adjustment section. The ECU will only access the idle tables when the TPS Closed flag is set. If this is not happening then it uses the main timing maps to determine IGN timing - this will not give us the 15 degrees we're looking for.
Without laptop: Nissan say to disconnect the TPS to lock the timing but I've not had a lot of success using this method. The other way is to hold the throttle open very slightly (TPS Closed flag must still be set) to increase revs a bit. So timing light in one hand, throttle in the other. You'll see the timing mark jumping around at idle and then when you increase revs to 1100 or so it will suddenly stabilise. This should be 15 degrees and this is when you take your reading. Don't rev it too much or you'll see timing start to increase.
With laptop: The only reliable method is to monitor the timing directly from the ECU via the Consult Port. You'll need a Consult Cable, a laptop and some monitoring software. I use Datascan software for this. Then you just watch the reported timing value live on the screen and adjust your Crank Angle Sensor until what you're seeing on the crank pulley (with your timing light) matches what's on the screen. Datascan also has a handy feature under "Active Tests" called "Base Idle Adjust" - this locks timing at 15 degrees.
Once you get a reliable reading, set the CAS (Crank Angle Sensor) to 15 degrees. This is a critical adjustment - particularly if your engine has been tuned for more boost etc. Less than 15 degrees will make it very sluggish. More gives extra power but may lead to detonation and subsequent engine damage. Note also that the timing marks start at -5 degrees so 15 is the 5th mark from the left. It pays to mark it with some white-out or brightly coloured paint. Base timing on most SR's is 15 degrees - a notable exception is the RNN14 which uses 20 degrees.