Well, as it turns out, harder than I thought! Apart from the fact that a straight seam seems to elude me, it's probably not a good idea to dash home from a morning's work and try and get three hours of cutting and sewing in before school collection time because that's when you cut all your new squares half an inch smaller than your existing ones. It's also why you sew some of your squares together both right side up and have to unpick all the results.
It's also a mistake to try and get any sewing done after school collection time, if you end up trying to arbitrate between 2 siblings having an argument about who didn't wipe their bottom and flush the toilet, and end up sewing in a strop.
Aside from all that, woohoo! progress is being made on the Siblings Together quilt!