You shouldn't have to do much if you're already in the interface.
Which interface are you using for ppl to connect to, wlan0 ? That's what you'll select then... put in two specific targets (Target 1 = the router, Target 2 = the victim), or leave them both blank to ARP the whole network.
Then click 'start'
Then open another tab and start something else like urlspoof, sslstrip, etc.