Oh crikey, its been ages since I set it up and I'm never 100% sure its all working correctly. As I recall you can pretty much stick to the defaults, the important part is figuring out what figure to set for the bandwidth limit to reduce buffer bloat.
I actually just checked and it looks like I DON'T have it prioritised at the moment on the Zen line but do on Plusnet which is interesting as BQM does indicate less packet loss on the Plusnet line.
Once its setup you just add a Firewall rule to classify ICMP echo request from the WAN into one of the high priority queues (display advanced at the bottom of the page, last option is Queue).