How do I add the htaccess code to solve the gtmetrix.com "The following cacheable resources hav

I am trying to speed up my slow WP multisite. I put in WP Super Cache and Autoptimize, and amazingly the site still runs. But I think there is supposed to be some additional htaccess code, that needs to go *someplace* in htaccess, to set future expiration dates. WP Super Cache generated a bunch of new htaccess code, which didn't break the site. I think I need to add something like this:
## EXPIRES CACHING ##

<IfModule mod_expires.c>
ExpiresActive On
ExpiresByType image/jpg "access 1 year"
ExpiresByType image/jpeg "access 1 year"
ExpiresByType image/gif "access 1 year"
ExpiresByType image/png "access 1 year"
ExpiresByType text/css "access 1 month"
ExpiresByType application/pdf "access 1 month"
ExpiresByType text/x-javascript "access 1 month"
ExpiresByType application/x-shockwave-flash "access 1 month"
ExpiresByType image/x-icon "access 1 year"
ExpiresDefault "access 7 days"
</IfModule>

## EXPIRES CACHING ##

Is this right, and where do I put it? At the end, after the supercashe, and after the WordPress? Within the Wordpress?

Thanks