Sample rate: 44.1 up to 96khz
Bit depth: 24 to 32bit
Peak-Headroom: -3 to -10 db
Bypass any applied limiter on the master before sending to mastering, and have a healthy headroom of -3 to -10 db in the loudest section. You can turn down or up the master fader to achieve this.
If there are plugins added after the mix that is intended for mastering purposes that makes it sound better to you, you might want to send 2 versions, one with and one without "mastering processing" However, no limiting.
If there are plugins on your master that was added early in the mix as a top down mixing approach, I want them engaged as they are mixed trough and all the mixing decisions are made in context of those processors. To clarify, any mixing technique is good as long as the desired result is achieved!
Make sure there are no unwanted clicks or pops in the mix that can be caused by unfaded audioclips or similar. Again, vinyl crackles and other desired and deliberate noises are more than welcome :)
Let me know if you have any further questions!