please post an tutorial how to use this plugin? no instruction at all?

Felix, after download and install, go to "Window > Extensions > Cut and Slice me", then all you need is described in the website: video, sample file and the text below.

• Organise your elements in groups, as if they were objects.
• Add @ at the end of the layer name you want to export, the content will be exported as a png trimming all transparent pixels.
• If you want your file to be a particular size, then create a rectangular vector as first layer and name it #. Your group will be exported at that size.
• Include "_BTN" to export all states in your buttons. Add groups containing your button states and name them as follows:
• normal
• hover
• clicked / pressed
• selected
• disabled