ADVICE NEEDED FOR NEWCOMER!

Happy Go Lucky's

O.K. so we are are new to caravanning and have learnt by our mistakes so far.  Just got to grips with the internal electrics/pump etc.  One year in and we have heard that we have to sterilise the water caravan pipes.  Now, what time of year is best for this. 
At the end of the season or the beginning?  In fact, can you give us an idiot's guide of what and how we should do this?