Mostly just by piecing together the members/properties defined in the chm, the few samples in that documentation, random posts here on Answers, and a little bit of trial and error.

Another tip, if you want to set certain claim values or force the user to reset their password, you have to register it first, the set the password expiration flag and call the update method separately otherwise it won't take, at least it wouldn't for me.

// register user
newuser.Register();

// set claim attributes
var claims = newuser.GetClaims();
claims.SetClaim(ClaimTypes.Email, new string[] { email });
newuser.SetClaims(claims);

// save user
newuser.PasswordUsesLeft = 0;
newuser.Update();

Wow, I am lost. For example I am looking for a delete user method, since create user is called Register, I selected the Unregister method. Again I get no information other than public int Unregister().

So I just try it,

testUser.Unregister()

It returns 500 Error. 

In order to learn an SDK from scratch, what I would be looking for is a description of what each method does and an example of it in use. Otherwise I am just going to keep getting 500 errors.

.Unregister() is the correct method

// connect to lfds database
Database = server.GetDatabaseByRealmName(Realm);
Database.LoginWindows();

var user = GetUserByName(recipient.Username);

if (user != null)
{
    user.Unregister();
}