Did you read the guide?
The best approach is to fall back to a backup from before this happened.
If you do not have a backup (now you know why you should backup often), then:
1. Make a list of all your plugins
2. Make a full backup of your current site, just in case
3. Download a new copy of WordPress from wordpress.org and a copy of each of your plugins.
4. Download a new copy of your theme.
Once you have that, via FTP delete all .php files (except wp-config.php) in the root of your wordpress site and all files in wp-admin and wp-include. Upload the wordpress files you downloaded from wordpress.org
Then delete all files and directories in wp-content/plugins and upload clean copies of each plugin.
Finally, do the same with your theme in wp-content/themes.
Next (finally? well, there's still more) change the password you use to admin your site for every user with a role other than subscriber.
Will this totally clean your site? maybe. :-)
Install the WordFence plugin. In its options, set it to scan everything and start a scan.