Step 1: Have your spine try to kill you in your sleep.

Step 2: Be unable to stand up long enough to shave for 3 months.

After that, the beard just kind of takes care of itself. :)

Edit I find it goes in stages:

1. Normal beard.
2. Amish farmer.
3. Hasidism.
4. Homeless dude.
5. Rasputin.

Right around stage 3 I need to think about trimming it back. :) I did hit full Rasputin during covid, that was interesting.

Hypothetical but as yet unexplored stages:

6. ZZ Top
7. Gandalf