How do I adjust the rear damping?

First, see here for definition of terms. Also, remember that Google is your friend. There are lots of more-or-less comprehensible articles out there about suspension tuning, including this one.

This article is using the Hagon aftermarket shock for an example, but the concepts are the same for all adjustable shocks. Just be aware that not all shocks have all kinds of adjustment. Some of the shocks that people use from other bikes (the EX500, for example) only have adjustability for preload and don't have any damping settings.

Generally speaking, when you screw the damping adjuster in, the bike rides more like a rock. When you screw it out, it rides more like a marshmallow. What you're striving for in your adjustments is a setting which is the perfect compromise -- not uncomfortable, but not so soft that the bike mushes all over the road every time you hit a bump.

The correct way to do this is to adjust it and go for a ride. Bring your screwdriver along. Stop every few minutes and fiddle. Adjust, ride, repeat. Make changes a little bit at a time. Either do a set number of clicks (like 1 or 2) per change, or do 1/4 turn changes, depending on how the adjuster works. The difference between smaller adjustments will be very hard to tell.

When you read your tuning article(s), you'll be interested to know that the Hagon's damping adjustment adjusts both rebound and compression damping, in a set ratio. Higher-ticket shocks usually have individual adjustments for these settings.

You should also keep in mind that taking someone else's settings and applying them to your bike is worse than pointless -- you weigh what you weigh, you ride how you ride, you prefer the feeling you prefer. The other rider does not. When you load their settings into your shock, it gives you this warm fuzzy feeling that you've done the Right Thing, and there's a danger that (if you're new to this suspension adjustment thing) you'll take however that feels and assume it's Right. It can be difficult or impossible to unlearn that, and it's most unlikely that their settings will be best for you.

Experimentation will give you the correct settings for you.