It's all in the Manual, but I'll post example anyway.
Let's assume your gateway's address is 10.0.0.1 and you configured web proxy on port 3128 and set it to be Transparent (ip web-proxy)
Now you should add following rule in ip firewall nat :
add chain=dstnat dst-address=!10.0.0.1 protocol=tcp dst-port=80 action=redirect to-ports=3128
This way, all requests to port 80 (http) will go through proxy meaning that your users will be forced to use http traffic through proxy no matter they want it or not.
Note that this is only for http traffic! You can't redirect all traffic to proxy!
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