Creating a Waterfall Release is quite simple:
If you want to create an EP with 3 singles and it is important to you that the singles come out one after the other and the EP "fills up", then you would have to proceed as follows:
You create single no.1 - and only this one and choose the desired release date.
You create single no.2 - and add the first single to the release (so a release of two tracks) and choose the desired release date. You can delete the first release.
You create single no. 3 - and add the two previous singles to the release (i.e. a release of three tracks) and select the desired release date. You can then delete the second release.
And so on and so forth, depending on how many tracks your EP/album has.
Attention: in order to keep the previous tracks in the new release and also to keep the streams, it is very important that the metadata is always identical. What is the metadata? The song title, the artists, if applicable the feature artists, the ISRC and also the audio files.