Follow these steps to use the ruler guides in PowerPoint to align objects between slides:

1. Open the presentation containing the slides with the objects you want to align.
2. Select the objects. You can select multiple objects by holding down the Shift key while clicking.
3. Select the View tab.
4. Activate the Ruler icon located in the Show group.
5. Move the objects (or slides) until the desired alignment is achieved.
6. Use the Ruler to check that your objects are aligned with each other across slides.