How to apply GUI Skins?

Hi guys, i want to apply a GUI skin to my login screen, one problem is that i apply it and it still don't work. Could anyone implement it, i just don't know where to put it. The Skins name is SASSkin

Here's the code, thanks:

 using UnityEngine;
 using System.Collections;
 
 
 public class database : MonoBehaviour 
 {
     public static string user = "", name = "";
     private string password = "", rePass = "", message = "";
 
 
     private bool register = false;
 
     private void OnGUI()
     {
 
 
     
         if (message != "")
             GUILayout.Box(message);
 
         if (register)
         {
             GUILayout.Label("Username");
             user = GUILayout.TextField(user);
             GUILayout.Label("Name");
             name = GUILayout.TextField(name);
             GUILayout.Label("password");
             password = GUILayout.PasswordField(password, "*"[0]);
             GUILayout.Label("Re-password");
             rePass = GUILayout.PasswordField(rePass, "*"[0]);
 
             GUILayout.BeginHorizontal();
 
             if (GUILayout.Button("Back"))
                 register = false;
 
             if (GUILayout.Button("Register"))
             {
                 message = "";
 
                 if (user == "" || name == "" || password == "")
                     message += "Please enter all the fields \n";
                 else
                 {
                     if (password == rePass)
                     {
                         WWWForm form = new WWWForm();
                         form.AddField("user", user);
                         form.AddField("name", name);
                         form.AddField("password", password);
                         WWW w = new WWW("http://sascommando.atwebpages.com/register.php", form);
                         StartCoroutine(registerFunc(w));
                     }
                     else
                         message += "Your Password does not match \n";
                 }
             }
 
             GUILayout.EndHorizontal();
         }
         else
         {
             GUILayout.Label("User:");
             user = GUILayout.TextField(user);
             GUILayout.Label("Password:");
             password = GUILayout.PasswordField(password, "*"[0]);
 
             GUILayout.BeginHorizontal();
 
             if (GUILayout.Button("Login"))
             {
                 message = "";
 
                 if (user == "" || password == "")
                     message += "Please enter all the fields \n";
                 else
                 {
                     WWWForm form = new WWWForm();
                     form.AddField("user", user);
                     form.AddField("password", password);
                     WWW w = new WWW("http://sascommando.atwebpages.com/login.php", form);
                     StartCoroutine(login(w));
                 }
             }
 
             if (GUILayout.Button("Register"))
                 register = true;
 
             GUILayout.EndHorizontal();
         }
     }
 
     IEnumerator login(WWW w)
     {
         yield return w;
         if (w.error == null)
         {
             if (w.text == "login-SUCCESS")
             {
                 print("WOOOOOOOOOOOOOOO!");
             }
             else
                 message += w.text;
         }
         else
         {
             message += "ERROR: " + w.error + "\n";
         }
     }
 
     IEnumerator registerFunc(WWW w)
     {
         yield return w;
         if (w.error == null)
         {
             message += w.text;
         }
         else
         {
             message += "ERROR: " + w.error + "\n";
         }
     }
 }