The rules are pretty simple. Your patch must have an area of 3 km2 maximum (3 x 1km squares). It doesn’t have to be rectangular, or comply with any OS grid lines; it can be any shape you want it to be. It just has to be 3 km2 or smaller. What it does have to be is contiguous (i.e. no floating bits that aren't joined on to the rest of the patch).
The birds that contribute to your score must be within the boundaries of the patch (i.e. you don’t need to be), or, seen or heard while you are on your patch. So, heard only birds count, distant passing seabirds count, flyovers count, and birds flushed from the patch while you approach your site count.
Unfortunately, to keep things in the here and now, we won’t be able to wait for rarities committees to verify records of rare birds. I should imagine the majority of rare or mega birds will be of suitable interest to other birders to attract some ‘external verification’, as will many of the scarce birds. However, more than one observer will see not everything so we’ll just have to rely on the honesty of the competitors.