I went to one of these with a mate a few years ago. There are about 30 odd people there and you get walked around the ground and the museum before being shown a bat-making demonstration by one of the guys from GN. He does a bit of bat shaping, answers a few questions and then gives you a form to fill out. This is basically how you get your bat. You have a choice of current profiles and stickers (apart from the Legend). You can specify the weight, sweet spot position, bow, and handle length/shape.
They then make the bat in Robertsbridge and post it to you a few weeks later.
It's a fun day out but it's not a bespoke bat making experience.