(for the Obsessive-Compulsive)
Assuming you've just gotten a brand-new engine and that the rev-limit were, say 7000rpm. Split that number in half, which gives you 3500. I myself actually started from 3000rpm. I then made 500rpm increments for every 500 kilometers. I'll use kilometers but you can find a suitable mileage for the increments.
At the beginning, all the moving pieces in the engine are still sharp at the edges. One of the key parts if not the most important one is the Apex seal which encloses the 'moving' combustion chamber. These apex seals are flat when in contact with the rotor housing. Since it maintains contact throughout its rotation at varying angles, the edges of that piece of metal get worn, and hopefully nicely rounded.
Compared with non-turbos, turbos create more powerful bursts of explosion that can be avoided when trying to get the new pieces to wear nicely. This is not to say it would render the engine useless, because at the beginning I myself didn't know how to run it in well. It still felt good, but compared with after, I think the engine noise (besides the exhaust explosion) is less 'gnarly.'
So how can it be done smoothly? Like the steering wheel, the gas pedal (accelerator) has what may be called a 'freeplay.' It is an initial area/span of motion before one feels a resistance. It's the 'soft' area before you feel the springlike resistance when you 'step on it.' An easy way to step smoothly is to make sure the needle in the boost gauge does not move drastically and stays under '+,' but do keep your eyes on the road! It's like driving a non-turbo as softly as possible.
Confining yourself to this freeplay, and within 3500rpm will ensure you're not adding sudden torque and at the same time you'd be breaking the car in to rev slowly, yet smoothly, without any 'ridges' from the bottom to the temporary rev-limit we are placing on ourselves. This also makes the car pick up slowly (crawl?) and painfully. Naturally, you'd be thwarting traffic and inconveniencing everybody around and yourself. It's also stressful. I did my run-in early in the morning or late at night (under cover of darkness).
As you continue this, you'll start noticing the engine noise becomes smoother but when you have no choice but to venture 'beyond,' there's a gnarly sound and resistance felt from the acceleration. You'd also notice that it gradually picks up faster within the enforced smoothened span. Beginners will notice the degrees of 'gnarliness' and should stay with the milder one whenever possible.
When the 500km are over, add 500rpm. Repeat the process within the newly 'appended' range. Meanwhile gradually add force when you step within the previous 'range.' In otherwords, you'd be 'ironing out' or smoothening (?) the gnarly resistance within the full self-imposed range minus the last 500rpm by stepping faster and faster while restricting yourself to the freeplay for the top 500rpm zone. It's basically revving to the 'previous top' and adding a short moment of 'lip service' before shifting.
The idea is to break it in as smoothly as possible with the minimum resistance from the engine at that point in its history and to make sure it revs smoothly from the bottom to the top without 'ridges.' Ridges are located where it's smooth at a point or points on the tachometer but gnarly in surrounding rpms. This is created when the engine is kept revving at a constant without smoothing out the range before and after. Highway use either forces one to stay at constant 'points' or to accelerate by stepping beyond the freeplay so I avoided it.
That's about it. Do you think I'm obsessive-compulsive? In my case, I started with 3000rpm because the turbo accentuates the torque band thereafter. Do remember to check and change the oil as necessary. Brand-new engines tend to burn more oil than stock engines that are already broken in. Some sports CPUs can be programmed to squirt more oil into the housing at higher loads accordingly.
A good engine indictor is when one can hear a faint yet clear 'high-frequency noise' from the engine. It cannot be heard as easily when the turbo(s) is/are blasting. I can usually hear this 'music' around 4000rpm and up when keeping turbo boost at a minimum.
I know some people who do not expect to have an engine longer than a couple years and who really want or need to get it 'broken-in' in the minimum amount of time. They'd get on the highway late at night and drive about 5 or 6 hours straight while observing the rev-limit they arbitrarily raise every X distance. They do this 'by ear' and have no choice but to ignore the gnarliness. They usually first get on the highway after doing their own preliminary break-in on streets for the initial segment, then they hit the highway about 4 or 5 nights to get the rest over with.