Well, the idea is to get them to start to flower early by giving them 12 hours of darkness. So as long as they're getting 12 hrs darkness, then it's up to you how and when you do it. I guess a lot of it depends on when it's practical or convenient for you to get out there to do the covering and uncovering. It's a hassle, and I'd only do it if you really have to.
Eg. If the sun rises at 6am, then you'd want to cover them each night at 6pm to get the 12 hours. Or, another way to do it would be to cover them when it gets dark, say at 9pm, then uncover them at 9am to get the 12 hrs darkness.
In reality, outdoor plants trigger with less than 12hrs dark, but by giving them a 'forced' 12hrs darkness, then you're assured that it's enough to trigger them into flowering.
Edit: If you do end up doing that, you must make sure that they're completely covered so that no light gets in. A heavy-duty black garbage bag, or even a plastic garbage pail would work, but again, it's a hassle and it makes the plants stand out like a sore thumb, so your individual situation will dictate whether that's practical or not. The easier way to do it if they're in pots would be to just bring them inside, or put them in a garage or something for the 12 hrs.