How do I make this PHP SQL piece of code vulnerable to sql injection

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SQL

PHP

Hi i need to make this piece of code vulnerable to sql injection any help please,its for a project im working on and i don't know much of either language.

<?php

$host="localhost"; // Host name
$username="root"; // Mysql username
$password=""; // Mysql password
$db_name="Faults"; // Database name
$tbl_name="Users"; // Table name

// Connect to server and select databse.
mysql_connect("$host", "$username", "$password")or die("cannot connect");
mysql_select_db("$db_name")or die("cannot select DB");

// username and password sent from form
$myusername=$_POST['myusername'];
$mypassword=$_POST['mypassword'];

// To protect MySQL injection (more detail about MySQL injection)
//$myusername = ($myusername);
//$mypassword = ($mypassword);
//$myusername = ($myusername);
//$mypassword = ($mypassword);

$sql="SELECT * FROM $tbl_name WHERE username='$myusername' and password='$mypassword'";
$result=mysql_query($sql);

// Mysql_num_row is counting table row
$count=mysql_num_rows($result);

// If result matched $myusername and $mypassword, table row must be 1 row

if($count==1){
echo "LOGIN SUCCSESS!!";
}
else {
echo "Wrong Username or Password";
}
?>