[How do you cut out objects in Affinity Photo?]

I have just downloaded the trial addition as my family have been unwell. I am tryin to establish how to cut out a with the cut out studio tool in X9? I have rasterised the image but can't get the selection brush tool to work as with the cut out studio there is no undo? it seems to like to cut out far too much? is there any way of ggetting round this as I want to purchase  the full versions but will stick with old faithful page plus if it isn't possible

Thanks