It took me a while to figure out the proper foot position. First off, it may be a misnomer to call it "heel and toe". For me it's more "arch and toe".
With just straight braking (when no "toeing" is necessary), I apply pressure on the brake pedal with the ball of my foot - it seems to give best control and modulation that way.
When "toeing" is anticipated to be necessary when braking for a turn, what I do is hit the brakes with the arch or "flat" of my foot - that is, the spot between the heel and toe - and I also angle my braking foot so the toe is angled over a bit, hanging over a bit where the top of the accelerator pedal is. Then, I find it works pretty well to rock my foot sideways to press down with the toe onto the top part of the accelerator pedal.
Since the accelerator pedal is hinged at the bottom, this works pretty well with fairly good control on the accelerator pedal motion, and the top of the accelerator is in a usable position.
Still, I do find that "Heel and toe" braking modulation ability (braking a bit angled sideways, using the arch of my foot) is a little bit less controlled than when I brake normally (with the balls of my feet), and this is enough reduction in braking control that I only do it when I'm planning to downshift and quickly power out of a turn. (I don't brake like that all the time - only when I need it).
And, I was just wearing some shoes today that I noticed I didn't like to heel and toe in at all, which fit a bit loosely. So, use some comfortable shoes (that aren't too loose fitting) to try it with. It is fun to do!
Does anyone left foot brake these cars, with performance enhancing effect? I have some front wheel drive cars I love to left-foot brake to get the most out of them (e.g., in autocross competition), but I'm not sure it would be effective technique at all in the RWD boxster.