Setting up NixOS for typical home SMB file sharing

configuration.nix

...
{

  services.samba = {
    enable = true;
    
    syncPasswordsByPam = true;
    # You will still need to set up the user accounts to begin with:
    # $ sudo smbpasswd -a yourusername

    # This adds to the [global] section:
    extraConfig = ''
      browseable = yes
      smb encrypt = required
    '';

    shares = {
      homes = {
        browseable = "no";  # note: each home will be browseable; the "homes" share will not.
        "read only" = "no";
        "guest ok" = "no";
      };
    };
  };
  
  
  
  # Curiously, `services.samba` does not automatically open
  # the needed ports in the firewall.
  networking.firewall.allowedTCPPorts = [ 445 139 ];
  networking.firewall.allowedUDPPorts = [ 137 138 ];
  
  
  
  # To make SMB mounting easier on the command line
  environment.systemPackages = with pkgs; [
    cifs-utils
  ];
  
  
  
  # mDNS
  #
  # This part may be optional for your needs, but I find it makes browsing in Dolphin easier,
  # and it makes connecting from a local Mac possible.
  services.avahi = {
    enable = true;
    nssmdns = true;
    publish = {
      enable = true;
      addresses = true;
      domain = true;
      hinfo = true;
      userServices = true;
      workstation = true;
    };
    extraServiceFiles = {
      smb = ''
        <?xml version="1.0" standalone='no'?><!--*-nxml-*-->
        <!DOCTYPE service-group SYSTEM "avahi-service.dtd">
        <service-group>
          <name replace-wildcards="yes">%h</name>
          <service>
            <type>_smb._tcp</type>
            <port>445</port>
          </service>
        </service-group>
      '';
    };
  };
  
}

@ddanon Thanks your solution worked well.
For anyone else having trouble, here's what I did:

1. skipped the extra config portion
2. Don't forget to setup sudo smbpasswd -a <user> to add a user. Also add a password to the user when prompted