Bukkit

import com.mewin.WGCustomFlags.WGCustomFlagsPlugin;
import org.bukkit.Plugin;

private WGCustomFlagsPlugin getWGCustomFlags()
{
  Plugin plugin = getServer().getPluginManager().getPlugin("WGCustomFlags");
  
  if (plugin == null || !(plugin instanceof WGCustomFlagsPlugin))
  {
    return null;
  }

  return (WGCustomFlagsPlugin) plugin;
}

import com.sk89q.worldguard.protection.flags.StateFlag;

public static StateFlag FLAG_NAME = new StateFlag("flag-name", true);

public void onEnable()
{
  wgCustomFlagsPlugin.addCustomFlag(FLAG_NAME);
}

import com.mewin.WGCustomFlags.flags.CustomFlag;
//you will need these imports later
import com.sk89q.worldguard.protection.flags.RegionGroup;
import org.bukkit.command.CommandSender;
import com.sk89q.worldguard.protection.flags.InvalidFlagFormat;

public class SampleFlag extends CustomFlag<SampleClass>
{ ...

  public SampleFlag(String name, RegionGroup regionGroup)
  {
    super(name, regionGroup);
  }

  public SampleFlag(String name)
  {
    super(name);
  }
  
  /**
  * the marshal method creates a YAML compatible value to save the flag
  * possible data types are String, Integer, Double, Boolean, Map and List
  * if you use Map or List WorldGuard will give you errors, but still work correct
  * Map and List produce error message if WorldGuard has the database activated
  * @param object the instance of the class to save
  * @return an YAML compatible value to save the flag
  */
  public Object marshal(SampleClass object)
  {
    return object.toString(); //can be anything that helps you to recreate the class later
  }

  /**
  * the unmarshal method recreates the class from the value you saved
  * the data type of object is the same as when you saved the flag
  * @param object the saved value
  * @return an instance of the class representing the value
  */
  public SampleClass unmarshal(Object object)
  {
    return new SampleClass((String) object);
  }

  /**
  * this method parses the input that is given from the user by /region flag
  * @param plugin the instance of WorldGuardPlugin
  * @param sender the CommandSender (player or console by default) who tries to set the value
  * @param input the String value that the sender uses to set the flag
  * @return an instance of the object that represents the input string
  */
  public SampleClass parseInput(WorldGuardPlugin plugin, CommandSender sender, String input)
    throws InvalidFlagFormat
  {
    if (SampleClass.isValid(input))
    {
      return new SampleClass(input);
    }
    else
    {
      throw new InvalidFlagFormat("The input format is not valid: " + input);
      //you can throw an InvalidFlagFormat to give the user a feedback why the flag has not been set
    }
  }

  /**
  * The method loadFromDb is basically the same as the unmarshal method, but it is used to save your flag to the
  * WorldGuard database. The only difference is that the parameter is always a String
  * @param string the string that has been saved in the database
  * @return an instance of the class representing the string
  */
  public SampleClass loadFromDb(String string)
  {
    return new SampleClass(string);
  }

  /**
  * as loadFromDb is basically the unmarshal method, saveToDb is basically the marshal method
  * it gives a string that can be saved to the database to represent the object
  * @param object the object that should be saved
  * @return a string that represents the object
  */
  public String saveToDb(SampleClass object)
  {
    return object.toString();
  }

import com.mewin.util.Util;

public class InteractListener implents Listener
{
  private static final StringFlag PLAYER_NAME_FLAG = new StringFlag("player-name");
  private static final CustomSetFlag INTERACT_ALLOW_FLAG = new CustomSetFlag("interact-allow", PLAYER_NAME_FLAG);
  private static final CustomSetFlag INTERACT_DENY_FLAG = new CustomSetFlag("interact-deny", PLAYER_NAME_FLAG);
...
  @EventHandler
  public void onPlayerInteract(PlayerInteractEvent e)
  {
    if (!Util.flagAllowedAtLocation(wgPlugin, e.getPlayer().getName(), e.getPlayer().getLocation(), INTERACT_ALLOW_FLAG, INTERACT_DENY_FLAG, "*"))
      {
      e.setCancelled(true);
      e.getPlayer().sendMessage(ChatColor.RED + "You may not use this here.");
    }
  }
...
}