Pretty simple, but you need a decent amount of strength in your wrists and hands to pull the thread through and lock it tight. An old lady with shaky fingers wouldn't be able to do it, for example.
(The process always used to make me think of Tim Severin's account of stitching together the leather boat "Brendan", and the massive muscles tightening in the saddle-maker's forearms!)

And you have to be careful about not missing a hole as you stitch, which generally doesn't become apparent until you reach the end and discover that the two sides don't match up anymore...

The Simple Way leaflets claim that you will 'make your first shoe in an evening and your first pair in a weekend', which is reasonably accurate. I imagine sandals would be much quicker, since stitching on the apron and back collar is the trickiest and slowest bit.

There always used to be a massive price difference between kits and ready-made shoes (about £40 I think) -- I would never have dreamed of paying for the ready-made version, especially when the whole selling point was originally that they sent the materials out as flat-packed kits. I assume they discovered at some point that there was a considerable demand for pre-assembled shoes from customers who didn't want to go through the whole 'make it yourself' process!