Sadly I don't get your question exactly... However, the reason why anyone would say you should record at -6dB's is just because you don't want any clipping of your audio signal.
I do a lot of orchestral music with virtual instruments for example. The software that's used is named Kontakt 5. In the settings I can choose if the instruments should be loaded by default with 0dB or -6dB.
If I would head this over to a mixing engineer, he would ask me to provide the stems with a maximum of -6dB to give him enough headroom to do his 'magic'. He will use a lot of plugins like Compressors, EQ's, Limiters which can and will make the track louder. In this case he has 6dB's of range he can work with. If he would throw all that on a 0dB track, it would just start clipping and you get unwanted distortion. He would also have no room to balance the tracks among each other if they're already at 0dB.
So, your question - How do you set it? - Well, just record quieter :P
If you record your audio with a microphone and your stereo out shows that your volume is at 0dB, then just turn down the volume of your microphone until you're at the volume you want.