Mandatory skills before ECU tuning

Lots of people want to tune petrol EFI engines for themselves

As modern and software oriented it may seems, it requires of people godly troubleshooting skills when things don't go as planned. Without all of those skills you will always depend on a skilled folk with dwindling patience who'll end up terminating your friendship with him.
Here are the most important to me

When creating something new it's rarely expected to work right the first time

Be the person who can take on challenges, not the one asking others to fix it for free and take the glory.

Buy yourself a fire extinguisher if is not done already.

General DC electrical skills

- master iron soldering, twisting wire is not allowed
- understanding the wire colour naming convention.
- understanding how fuses work and enjoying their benefits
- understanding how relays work, there will be an awful lot of them ahead
- understanding your engine wiring diagram perfectly, if you can't say why it is rigged this way try again.
- being able to use a multimeter to read voltage,impedance,frequencies and capacitance.

IGNITION skills

0- understanding engine firing order and being able to place spark plug wires accordingly
1- being able to test for spark events at the spark plugs (getting a spark occurring outside the cylinder head for tests)
2- being able to tell when an engine does not run on all it's cylinders and locate which ones
3- being able to read spark plugs colours. (being oily, rich, lean, and more importantly variance accross cylinders)
4- being able to properly install a timing belt or timing chain
5- know what is ignition advance and ignition retard.
6- being able to use an ignition timing light
7- being able to locate TDC on the reference piston on an engine with a test rod(making sure the reference point on the harmonic dampener is true)
8- being able to adjust an ignition distributor

FUEL INJECTION skills

- understand what composes the fuel delivery system and identify which part on your car is each one(appearance varies from one implementation to another)
- identifying fuel leaks and fixing them appropriately
- being able to measure fuel pressure with a gauge
- understand how to adjust a fuel pressure regulator if needed
- knowing the various smoke colours that can come out of a tail pipe and their origins
- knowing what is a rich and lean mixture
- detecting various bad analog sensor readings
- being able to test injector impedance
- understanding how the universal narrowband oxygen sensor works and testing
- understand how to install and read a wideband oxygen sensor/gauge
- rough injector operation testing
- understand what the injector duty cycle means ,measuring/calculating it would be even better

ENGINE MATH THEORY skills

- understand the concept of engine volumetric efficiency ( VE )
- understand the concept of [RPM x VE ] and  [RPM x pressure] mapping for fuel and ignition
- understand what is engine air flow
- understand what an air flow meter outputs and interpret it's reading
- understand what forced induction does to engine air flow

SOFTWARE use skills 

- know what is a COM port on a laptop computer
- being able to find the COM port # you'll be using when tuning. those things change from time to time :)
- being able to use the data logging interface offered by the ECU,if available
- being able to locate data log files on your computer and read them with a spread sheet software

With those skills you should be able to troubleshoot many basic engine related issues and should avoid being a pain in the ass to someone else who does know. there is far more to be known than this, If all of this is already boring, don't bother going deeper. :)