Refer to the documentation included with your rental for the recommended methods. It is highly recommended you follow the cue number scheme, which corresponds to cue numbers listed in the NOTES for each slide (NOT the PowerPoint slide number!).
If you are only cutting slides, and not adding them, you can hide them instead of deleting them. The slideshow will simply pass over the slide you “cut” in this way. To hide a slide, right click on it and select “Hide Slide” from the bottom of the dropdown.
When looking at the slideshow in Presenter View, you will see the notes field on the right hand side of your screen. That is the cue number which ties to the marked piano-vocal score you received.