You can add new slides easily with a single click and then choose the slide layout that you want to apply to the slide. If you decide after the fact to change the slide layout, no problem; just click the layout that you want and PowerPoint does the rest. (You might need to move the slide elements to where you want them on the new layout, though.)
Click the Home tab.
In the Slides group, click the New Slide arrow.
A gallery of slide layouts opens.
Choose the layout that you want to use for the new slide.
Alternatively, if you want to make a duplicate of a slide you’ve selected, choose Duplicate Selected Slides.
You can reuse slides from another PowerPoint presentation by choosing Reuse Slides ...