I wanna create a custom login form template

You don't need to do anything special. You can copy the form HTML on wp-login.php and simply post your data to that page.

<form name="loginform" id="loginform" action="YOUR_DOMAIN/wp-login.php" method="post">
    <p>
        <label for="user_login">Username<br>
        <input type="text" name="log" id="user_login" class="input" placeholder="Your username" value="" size="20"></label>
    </p>
    <p>
        <label for="user_pass">Password<br>
        <input type="password" name="pwd" id="user_pass" class="input" placeholder="Your password" value="" size="20"></label>
    </p>
    <p class="forgetmenot"><label for="rememberme"><input name="rememberme" type="checkbox" id="rememberme" value="forever"> Remember Me</label></p>
    <p class="submit">
        <input type="submit" name="wp-submit" id="wp-submit" class="button button-primary button-large" value="Log In">
    </p>
</form>

In the future, please use search before asking. This question has been asked multiple times.