All I did to make it into a nursing top was turn it inside out and cut a long slit down the middle of the layer that touches my skin and I was done. That is it, just one cut.
However in my tutorial I added the ribbons because some people might need them, depending on bust size. I never used them. I still use the top years later, it worked very well for me.
If you do not have a shirt that is 2 layers then you can sew two shirts together making your own 2 layer shirt. If you would be interested in this type of tutorial let me know.