How much brandy and mushrooms? They are missing from the ingredient list.

When online instructions are missing something, the first thing to do is to check to see if it was posted elsewhere on the Internet.

A quick inquiry (using the terms "Steak Diane With Miyazakigyu Wagyu Ribeye double8") on a search engine (yeah, one of those websites people used to use 10-15 years ago) brings up this page as the #2 hit:

https://cookpad.com/uk/recipes/15209913-steak-diane-with-miyazakigyu-wagyu-ribeye

which calls for 1/2 lb. button mushrooms and 1/2 C. brandy.

Note that this type of Internet sleuthing isn't exclusive to food recipes. It can be used for any set of perplexing instructions: auto repair, Linux system administration, knitting, whatever.

Anyhow, best of luck.