The first step is to change to Slide Sorter View. Then, select the slide you want to copy from. Choose the Format Painter button from the toolbar and then click on the slide to which you want to copy. It is a good idea to put a blank slide at the end of the slide show. Otherwise, you would return to the Slide View at the end of the slide show.
Some people use a company logo instead of a blank slide.
Another way is to select the E nd with black slide option in the Options dialogue box. You will find this option on the View tab.