George was in for almost a month after neutering, I made sure to take him out on his own (no distractions from the others) on a harness and lead. We went very slowly as far as we could while keeping the house in view, whenever a car, person or other scary thing came along we both bolted back to the house and came out again when the coast was clear.
Two days of this, front street and back garden, chasing back through the (pegged open) catflap or open front door until I was confident he know his way home from all angles, then he was allowed out with his pal Kuki who showed him all the places her hooman doesn't know about.
I'd let your calmest cat out first, have him do the reccies, then one by one let him show the others around. Definitely get the flaps sorted out, though, I'd be looking to let them out before the really bad weather lands, just in case one escapes before you're ready.
Yes, Gill made the most important point, when you let them out on their own for the first few times, make sure they're good and hungry - that way they haven't got a full tummy to go exploring on!