Not completely sure what you mean by "conditions", but essentially you list up to six preferences for halls (minimum of three), in order, of which only one can be a 30-week ensuite and only one can be a 39-week ensuite. This is because ensuite applications are massively oversubscribed (and with Rootes becoming 39-week lets, I would guess this will become even more so now). Also, giving only three preferences with two of them en-suites won't increase your chances of getting an en-suite; if anything, it will just make life harder for you in the event of not getting an en-suite, as if the non en-suite choice is oversubscribed you might get put somewhere you really don't want.
The deadline is July 31st to be guaranteed accommodation with the university - everyone that applies before this date is on an equal footing. So don't rush to do it the second the application form goes live, because there's no point.