When you open libreoffice impress you will get a dialog to select a templete with a preview select which one you want to use by left clicking and then click open. To add a title to a slide click where it says click to add title.
To open a previous presentation you have been working on press the button that looks like a folder with a piece of paper in it and navigate to the file you want and open it. To save your presentation press the button that looks like a floppy disk.
