My setup looks essentially like this:

{
  # Enable the KDE Plasma Desktop Environment.
  services.displayManager = {
    plasma-login-manager = {
      enable = true;
    };
    autoLogin.enable = false;
  };
  services.desktopManager.plasma6.enable = true;

  environment.systemPackages = [
    # provide onscreen keyboard
    pkgs.kdePackages.plasma-keyboard
  ];

  programs.kdeconnect = {
    enable = true;
  };
}