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This is very simple to make, but people tend to be confounded by it because it's so different from modern pants.
Rotate the gusset 90" from how it's drawn, and connect the large angle with where the small angle was cut out (at the crotch). and the small angles at the ankles of the legs. Each side of the diamond goes down the legs.
If you use this pattern on DA, I'd like to see what you made.

Anyway, I made up a pair over the weekend. I had to consult a more experienced sewer to explain what you meant by "turn the gusset 90 degrees", but once she did it was easy-peasy! A little hard to wrangle up that much fabric around the waist to get the drawstring pulled, but once it is, no worries. Very flowey and terribly comfortable!
Thanks so much for the pattern! Saved me a nice chunk of change!