How to automatically start a slideshow

Running Impress, how do I start a slideshow automatically?

do you mean:after opening impress and loading the presentation, how to start show from first to last slide?

In that case in the menubar: slide > transition > a sidepanel is opened, at the bottom you find

advance slide > select automatically > adjust time > apply to all slides

F5 should now start the auto presentation. Manually adjust times for one slide to vary presentation time for that slide only