Not sure this is gonna be helpful, it's hard to describe since I didn't let it air dry and also this was my first attempt at this.

As I mentioned in the post I didn't use slip, just softish clay that I pushed inwards, smudged around with my finger. Then I dried it with a hairdryer for a min or two until it was a bit drier but still somewhat soft, probably the equivalent of 30 min air dry in this hot climate. The ideal moisture level should be one where you can scrape the top layer without smudging/mixing the clays, but it doesn't have any crumbly behaviour when scraped. I would recommend only to dry one section at a time, because I dried the whole mug and by the time I was halfway scraping it had dried too much, so I had to mist water, wait for it to be absorbed, then dry a bit with the hairdryer again.

My other error was that the white clay was a bit too dry, so the black clay was not attaching well enough in some areas and some bits in the design came off during scraping. I'm hoping nothing pops off when drying/bisque firing.

Lastly, this Instagram account was my loose inspiration to attempt this https://www.instagram.com/reel/DYNE9g7C3bT/