Check for Leaks
If your Megasquirt is installed correctly, when you turn the ignition on you should hear your fuel system prime. It is very important to check for fuel system leaks at this time. You could find anything from a drip to a stream of fuel. It is extremely important to fix these leaks before proceeding. Take caution when fixing leaks as pressurized fuel could spray out. Cycling the key a few times when you believe the system is tight would be a good idea. A few fuel pump priming pulses should be a good test before getting the car running.
Connect to Megasquirt
Install TunerStudio on your laptop. Then connect the serial port from Megasquirt to your laptop's USB port using the supplied cables. At this point, follow the instructions in the Megasquirt manual to finish connecting. You can find the instructions for this in section 2.10.
Go through your dashboard and make sure sensors are reading correct values. If the specific sensor is not shown on the dashboard, you can add it by right clicking on the dashboard and adding a gauge. Your intake air temperature should be reading whatever temperature it is where you are working. Coolant temperature should be similar to the intake temp. If any sensor is not reading correctly, check that you have it wired properly. Sometimes a bad ground can cause sensors to read strange values.
You must also make sure that the timing is synchronized between the engine and the ECU. If you haven't already, set the trigger offset for your trigger wheel. For most installations, set the #1 Tooth Angle to 80 degrees. This can be found within the ignition settings page of TunerStudio. Also set the edge selection to Rising Edge while you're in there.
At this point you will want to verify timing. Make sure your injectors are not connected or your fuel is turned off. Then set your timing to fixed 10 degrees in the ignition settings. It should be on the right side near the top. Have a friend crank the car while you check the timing with a timing light. Make small adjustments to the #1 Tooth Angle until the timing on the crank matches 10 degrees. It should already be fairly close, if not dead on. Make sure to reset the timing from fixed timing to use table once you set the timing.
If at this point everything seems correct, you have oil pressure, fuel pressure, and no leaks, go ahead and start the car!