Dove inserirlo dipende da te da dove vuoi il log-in;
Cosa inserire (grossomodo questo):

*** <?php
global $user_ID, $user_identity;
get_currentuserinfo();
if (empty($_COOKIE[USER_COOKIE]) ) {
?>
<form name="loginform" id="loginform" action="<?php echo get_settings('siteurl'); ?>/wp-login.php" method="post">
<div><label><?php _e('Username') ?>:<input type="text" name="log" id="log" value="" size="20" tabindex="7" /></label>
<label><?php _e('Password') ?>:<input type="password" name="pwd" id="pwd" value="" size="20" tabindex="8" /></label>  
<label><input class="check" type="checkbox" name="rememberme" value="forever" tabindex="9" /><?php _e("Remember me"); ?></label>  
<input class="submit" type="submit" name="submit" value="<?php _e('Login'); ?> »" tabindex="10" />
<input type="hidden" name="redirect_to" value="<?php echo $_SERVER['REQUEST_URI']; ?>"/>
</div>
</form>
<?php
}
else {
?>
<b>Bentornato <a href="<?php echo get_option('siteurl'); ?>/wp-admin/profile.php"><?php echo $user_identity; ?></a>!</b> - <?php wp_register(); ?><a href="<?php echo get_settings('siteurl') . '/wp-login.php?action=logout&redirect_to=' . $_SERVER['REQUEST_URI']; ?>"> - <?php _e('Logout'); ?></a>
<?php
}
?>***