I assume you are talking about metro beaches... there are some great downwinders further afield... but as far as metro here are a few tips:
Henley to Kiteshop is a good run. 2 x jetties, usually some good flat gutters along the way and a few kickers.
Henley North Sailing club to kiteshop. 1 x jetty, again some good flat spots and kickers
Anywhere from north of grange jetty to Bower rd.
Generally the swell will get smaller as you travel north. Anything further north than bower rd is pretty flat

(also run into council restrictions). You can pretty much pick any spot and go from there, perhaps try from north of grange jetty up to bower rd carpark for a first session so you dont have to navigate jetties on your first go.
Give swimmers and crowded areas such as Henley square a wide berth on your way through, and finally, watch out for fishing lines as you go around jetties cause squid jags and kite lines dont mix........