To use libreoffice writer type what you wish to type into the main part of the window. If you wish to save your data press the icon that looks like a floppy disk or press control + s. To open a previously saved document press the button that looks like a folder or press control plus o. To load or save a file from the menu file -> open to open a file or file -> save. To have save your file as a pdf press the button that is a sheet of paper with curved red lines on it or go file -> export and select the file type as pdf.
To change the font size There is a drop down box with a number in it you can type the font size you want in this box or select from the dropdown menu. You can change the font with the drop down box to the left of the font size which you can type and it autocompletes the fonts or select from the drop down menu.
To check the spelling of your document press the ABC button with a checkmark, from the menu -> tools spelling or press f7. After you are done writing your document you can press the button that looks like a printer to print, press control+ p or file -> print.
