There are easy to follow docs on setting up the proxy in RouterOS. Just make sure you create a firewall filter rule to block access to port 8080 coming IN on your internet interface. Also, you will need to create a firewall nat rule to forward requests for port 80 to port 8080. This will create a transparent proxy - one that does not require an end user to configure a proxy server in their browsers.
I use the Web Proxy for my hotspot setup. It is invaluable. I would configure it to not cache anything away as the RB433 does not have a lot of breathing room for that sort of thing. I use it to block at the DNS name level, file extensions, etc.