This article goes through how to set-up your Swimming training zones based off your CSS (Critical Swim Speed).
First you will need to complete an CSS test, which is two tests consisting off a 200 meters effort and a 400 meters effort. The CSS test take less than an 40 minutes, the protocol used in the BCA training plans is below.
100 m @ Z1/2 w/ 5 secs rest + (FS = Freestyle)
050 m @ Z1/2 w/ 5 secs rest + (PU = Pull (buoy between legs))
050 m @ Z1/2 w/ 5 secs rest + (KB = Kickboard (only legs))
050 m @ Z1/2 w/ 5 secs rest (FS = Freestyle)
Rest 30-60 seconds
200 meters @ max effort
Rest 3 minutes