“Just slouch forwards pushing your knees together.”
Good advice and very bad advice in one sentance! Slouching forwards is the best way to describe a good position for skiing but pushing your knees together is bad technique and bad for your knees.
You hip, knee, ankle should remain in a straight line. Its the fact the ski is edging that allows a snow plough to slow you down, many cr@p instructors fail to tell you this hence the bad technique.
When I taught Mrs FD to ski last year I found snow ploughing very tiring, basically you are braking all the time. In a snow dome you probably dont have the space, but I ended up teaching her to do a snow plough turn followed by long traverses across the slope, at least that way your legs get a rest, plus it gets you ready quicker for pararell turns.