The SBY is 'inserted' as any other duty, one day you may be flying, the other day you are standby... quite simple. Just the same way probably as they plan it at your company.
easyJet has something called a reserve period where the idea is that for about a month twice or three times a year, the 5/3/5/4 roster is supsended for some crewmembers in the favour of a random roster, just to cover any problems of a fixed roster. At least, that's what they say it is for
In practice it means you fly more during your reserve period, and you still get standbys during your normal roster pattern. Confused? Me too.