Firstly I 'trued' up a corner of the felt so I had a decent right angle. I then measured the position of the first column of ribs and laid them out. These were laid half on and half off the edge so that the final seam would be in the middle of the bottom side and not at a corner. Next allowing the same gap used on the sample bellows corner I had previously made, I laid out the 2nd and subsequent columns. All ribs were stuck down using white PVA glue.
(Note: The more accurately you lay out the ribs - the easier it is to fold up the bellows. I did it by eye - and it was good enough - but I'd take more care if I was doing it again and measure more carefully).