Here's a guided exercise [1], and a video with more explanation can be found here [2]. You shouldn't forcefully exhale completely; just let go of your breath. Also, sit or lie down; don't stand or do anything else.

Initial scientific evidence related to auto-immune diseases here [3], although please don't mistake interviews with Hof with science. For example his comments about "charging your body with oxygen" and "alkaline blood" are completely off; a proper explanation can be found here [4].

[1] https://www.youtube.com/watch?v=tybOi4hjZFQ

[2] https://www.youtube.com/watch?v=RpUkR47dmjU

[3] https://www.pnas.org/content/pnas/111/20/7379.full.pdf

[4] https://www.daniel-timms.com/wim-hof-method-biology-of-breat...