Thanks Dave, but this is where I think there's confusion: if I do exactly as you suggest, the first part of the two part instruction doesn't leave me with two pieces, but rather with a grid with a 7-cell wide hole in it (or, if being even more literal, a grid with two slots).
This is why I've become more confused when I read people saying they figured it out by following it exactly (emphasising the exactness): one has to interpret the instructions
less precisely if one is to have it apply to the whole width of the grid. If one applies the instruction to the 7-cell strings (or to the 3-cell strings which are perhaps implied elsewhere in the puzzle) then not enough is achieved.
I don't wish to argue btw, and I can see a final grid that is surely what's wanted (five correct columns included), but I'm not satisfied that the instructions lead me from here to there -- which makes me think there could be something in the details that's important, and which I just can't see. Not understanding that final step can sometimes be an indication that it's actually completely wrong. 😁