In Excel, you can create a button that will center your selection across the page. This can be done by following these steps:

1. First, open up Excel and select the cells that you want to center.
2. Then, go to the 'Home' tab and click on the 'Center' button in the 'Alignment' section.
3. Your selection should now be centered across the page.
4. To create a button that will do this automatically, go to the 'Developer' tab (if you don't see it, go to 'File' > 'Options' > 'Customize Ribbon' and check the box next to 'Developer').
5. Click on the 'Insert' drop-down menu and select 'Button'.
6. Draw your button anywhere on the worksheet. In the 'Assign Macro' window that pops up, select the macro that you just created and click 'OK'.
7. Now whenever you click on your button, your selection will be centered across the page.