Hello Reader's! If you are new to Wordpress and want to make a login custom page, then you can see an example below:-

First you have to create a page on the root, name it login.php

<?php
/*
* Template Name: Sign In 
*/

if($_POST['submit'] == 'signin'){
$username = $_POST["username"]; 
$password = $_POST["password"]; 
 $mysqli=mysqli_connect('localhost','root','admin','wordpress') or die("Database Error");
$sql="SELECT name FROM user WHERE email =  '".$username."' and password = '".$password."' ORDER BY name";
 $result = mysqli_query($mysqli,$sql) or die(mysqli_error());

 if($result)
 {
  while($row=mysqli_fetch_array($result))
  {
   echo $row['name']."\n";
  }
 }
}

get_header(); ?>
<section>
     <form method = 'post' action ="#">
<input type="text" name="username"> 
<input type="password" name="password"> 
<input type="submit" name="submit"  value="signin">
      </form>          
</section>    
<?php
get_footer(); ?>

Now publish this page by using the template type = signin

Now as you can see the this page self having the login form and sending the two param username and password to itself. Now we can use mysqli_query function to get the result on this page only and you can code according to you.