How to manually trigger a workflow via a button in the Ribbom?

I’m running into a problem, I need to loop through all items in a list, and when a certain condition is true (a yes/no column will have the value “yes”), I need to do a calculation.
I have the calculation in place, the conditions are in place, even a (reusable) workflow on a single item works, but my client would like to have one button in the ribbon which he can click and then the workflow will go trough all items in the list.

Please help! Thanks

PS: working with O365 and SP Designer