For IIS, you simply have to follow these steps:
– upload and activate HMWP plugin
– in HMWP settings, start tab, select ‘Medium Pripacy – More Compatibility’ scheme and save settings.
– in start tab click on “IIS configuration” button and follow guide to update web.config file. (web.config is located in WP root directory and you should use FTP to update it. Note that you may need to update this file whenever you change settings or install a new plugin or theme)
In your case, after saving settings in HMWP plugin, the CSS breaks. This means HMWP plugin has actually masked/changed the wordpress default paths, but the rules you have added in your web.config file is not working somehow.
Can you please confirm with your host the web.config file location? Also can you please send us your web.config file before and after adding HMWP rules, as zipped file.