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Step 1Press the allowances to the wrong side.
Step 2Mark where the edges intersect with pins (or diagonal clips).
The process will work when the two allowances are not the same width. When the allowances are different widths, the fabric corner that forms (in Step 4) will be uneven and asymmetrical.
I'd read about this technique somewhere long ago. But, seeing it demonstrated on the Threads DVD clarified it and brought it to the front of my brain.