Using l3keys module for a command

[uzibalqa]

I'd like to understand how to use the l3keys module.  

After defining a set of keys with

\keys_define:nn { mykeys } { ... }

how do I write a command that applies these keys?

For example, I have the following command:

\NewDocumentCommand {\bh} { o }
  {
    \tl_if_single:nF {#1}
      { \keys_set:nn {mykeys} {#1} }
  }

I'm not entirely sure how to make this command accept key-value 
options from the user and apply the definitions I set up with 
\keys_define:nn. 

Could it be explained how to properly structure such a command so that
it processes keys both from the user input and from the predefined key 
set?

A minimal working example of how to define keys and use them in a command
would be important.