I do heel toe all the time when I downshift.
How I do it? It comes naturally for me, and maybe my average Asian build (5'6") helps.
Brake with your toe, then rotate foot anti-clockwise with toe as the pivot point, tap throttle with your heel. I don't keep heel on the gas pedal, I just tap it once of twice to blip it.
Hope this helps.
E30 318i M42B18