Thanks for your comment, that's really useful feedback!
It's tricky to update the instructions on live tasks, but for future ones, would a version of the text below be helpful?
'Draw rectangles around the titles of plays featured on the next performance date. When you're happy with the outline, choose the green tick to save it. Most playbills list more than one play for an evening, so mark all you can see. Please include subtitles (alternative titles, like 'or the something or other') where they exist. Ignore any performances on past or future dates. You don't need to include song titles. Press save/submit when you're finished marking up a page.'