There are a some things to make sure of.
1. Always run the engine at 200+ degrees. Get it their right away. If you have to cover the head then so be it.
2. Never, Never bring the engine up on the pipe (to max RPM) on the first 3 tanks of fuel. Your rod, crank and wrist pin are not ready for it.
3. After it will hold temp at 200+ then you adjust your low needle and idle settings.
4. Just drive the car. Don't drive off road. Keep some tires handy for break in that you can run in the parking lot. This way you do not stress the engine when it is mechanically tight.
5. If it is cool then lean it out. Take your throttle input up to just before it gets on the pipe and set your high needle so the motor will hit 200 degrees with short burst of the throttle.
6. You do not have to let the motor cool before it is broken in to a certain point. It is not about fuel, it is about quality run time at 200 degrees where the engine was designed to run. Once it will hold the same temp for 4 - 5 minutes of good running and you have it tuned to idle correctly, you can shut the engine off.
7. The heat gun is your friend. Most engine wear comes from starting a cold engine.
8. Always - Always make sure the engine is stopped and cooled with the piston NOT at Top Dead Center. It will kill a motor's life span.
A perfect break in will result in you starting the engine one time, running at 200 + degrees, getting your idle set, watching the motor change and then hold the temp at 200 + as you drive it a little more agressivly each time you fill her up. Then you kill the motor the first time. Usually this is after 4 - 6 tanks depending on how hard you run it. Thermal cycle (up to 200 + then back to room temp) 3 to 4 times at one tank each time then head to the track.
You are ready to race tune and the parts will be lapped in correctly at the temps the manufacturers designed the engine to run it at. At first you will lean it out quite a bit. Then, as it loosens up you will richen it to keep it cool.
When you don't have to richen it up any more for it to stay stable, you are 90% there.
It is all about metals and how they work.