how can i convert a .cda file to an mp3 file

You can't.

.cda files are simpy pointers on an audio CD to the beginning of a track.

A CD is only continuous track like an LP record.

If you have an audio CD, insert it into your computer and use iTunes to import the audioCD.

you'd better convert your CDA files to the most common mp3 format. To know more about how to do that, you can refer to this article, hope it helps!

Irene, It is not possible to convert a CDA into an audio file. As the article you referenced makes clear, it is necessary to have the original audio CD.