Custom White Balance?

I'm wondering for a few days and searching for an answer. I'm using Nikon D5200 and wanted to PRE AWB (Custom White Balance)

Now my question is - What setting do I use to take the white/18% gray picture?

I mean by "settings" is what distance from the white object, what initial AWB the dial should be set to? Also what Aperture and Shutter Speed should be set to? Should the flash fire?

Hope I described my question clearly.

Whenever I have a question/problem such as yours, I read the user's guide
(aka "the manual").
It is amazing just how much I always learn from that.

If that doesn't help (or if I'm interested in a product I don't own), I check the manufacturer's website.
They have descriptions and specs for their gadgets, often have user forums, and the decent manufacturers all let you download the manuals in electronic form (so losing the book is no longer a problem).
Getting the PDF is also useful for those times where the printed book is rather small and useless - the PDFs often have more detailed information.

99% of the time reading the instructions does the trick.

Do yourself the favour and read the manual - you'll learn a lot and you'll get much more enjoyment from your camera. Then go get a few books on Photography - you'll LOVE what that will do to your results.

If you don't understand all these features, why complicate things by using Custom WB?
Just set one of the pre set options for whatever lighting conditions you are using.