I would strongly second the wise words offered above by Bob Bolton " Be warned"... " to use quality card" This constituted the area of my big mistake when I long ago made new bellows for my Lachenal Anglo. It is the card which gives the bellows its structure and the card used needs to be able to take a lot of stress. When I made my bellows I used card left over from making some school work cards and over the years it has not completely stood up to the job, although it has done quite well for a long time. My bellows is now over twenty years old and still doing quite a good job, but it can get 'dimples in its cheeks' on a big pull of air because the card may buckle a bit under the strain. If anyone can here describe exactly the type of card most suitable for the job then I would also be grateful because I would like to build a new set of bellows sometime. I have toyed with the idea of trying to replace the card with thin sheet plastic- in fact I did cut out a complete set of plastic trapezoid blanks to do the job but I have had lingering doubts about whether the plastic would prove to be too hard and sharp and so might cut through the leather covering it as the concertina is played.
On the question of the leather - I cut all of the many pieces needed from an old leather coat which I bought for a few quid in a charity shop. I had to skive, or thin down, the narrow strips which are used to join the folds but otherwise it was no problem and it has given good service.
Alan Day refers to a "collapsible wooden jig" which is the concertina shape around which most bellows makers construct their bellows. perhaps like this HERE
I did not find out about this device until after I had finished building my bellows by just measuring carefully and sticking all the bits together so that they looked right. That worked for me. The glue I used was an impact adhesive which is clearly not what the Victorian builders would have used but again it worked for me.

Good luck with the project.
Regards, Geoff