I cut from the bottom up, the top has wires in the hold the leather down (not sure if the cloth seats are the same, but I couldn't bothered pulling out more hog rings - the 18 you need to remove the cover/foam from the base is more than enough).
I also initially cut mine crooked like he did and thanks to his mention of it was paying enough attention to notice and resolve it before re-assembling the seat:

(I actually used a hand saw, then scraped across with a bread knife to level it out, instead of initially using a knife because it looked quicker and I didn't have to go inside to get it!)

Vision is heaps better now, I'd posted a photo before (I think) showing how badly the visor affected my vision, I might actually re-install the passenger visor now


Before:

Foam before modifying (I didn't take one after I was done, unfortunately, but I just cut off the whole hump section.

This shows the bucket area of the seat for anyone interested in doing it.

The bottom of the seat with modified foam:

It's amazing how much difference it makes; I squeezed the removed section between my fingers and it compressed from just over an inch to less than 5mm; but I guess the best way to explain it is that's point loaded, when you sit on the seat it's spread over the entire foam so it makes more difference than you'd imagine. I have an occasionally bad lower back, and the seat previously hadn't felt great; now it's much more supportive. Initially it feels odd because it's different; but the description of it being like sitting in a big baseball glove sort of makes sense.
The armrest makes more sense too now (and the area around it) and my hair doesn't touch the headliner unless I'm looking around, my legs aren't quite correct in my work shoes (which have reasonably thick soles) but it's much better in bare feat or normal flat shoes than it was before; in terms of not running my hands into my thighs while turning the wheel through it's full range of motion. The seat back seems to hold me better (just by sitting that bit lower) and you can tell you're more out of the wind. Previously to experience this (especially getting down out of the wind on colder nights at 100kph) I had to hunch down in the seat and push my butt forwards so much that my legs actually came up high enough to contact the back of the steering wheel. I'm closer to center of the windscreen too. I'd actually consider a roll bar for the car now (previously I'd seen no point, since in an accident my head would have been sticking past it anyway)

Because I didn't cut from the top of the seat base, the shape of the leather shouldn't get effected too much, and because it's tight around the bottom and the whole lot (foam, leather and all) moves down as you sit on it, it shouldn't really stretch much with any luck. I've seen some good tricks involving steam to tighten leather back up anyway if it becomes a problem.