You should be able to hit CTRL-S to quicksave (and CTRL-L to load the most recent quicksave - the game keeps 2 by default). Hit S to bring up the save window (or return to the menu and select save there), click in the text box at the bottom of that window (once you have saves you can also select one to overwrite), type in a description and hit the checkmark button to save.
You can save at any point in the game, except during cutscenes and conversations (and one smallish cave where the cutscenes at the beginning and end are connected).
There is a link to the pdf manual in the Start menu folder for the Larian Vault release (the file itself in the '..\Divine Divinity' folder. With the Steam version it is located here: '..\Steam\steamapps\common\divine_divinity\dd_manual.pdf'.
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