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I believe ISO-8859-1 will support everything in French apart from the ‘oe’ ligature. So you should be fine with just cutting and pasting your text directly into an editor that can save out as 8859-1 (which is most of them).
However if possible I’d suggest you consider using Unicode (with a UTF-8 encoding) instead. Not only does it natively support the missing ‘oe’ ligature but will future proof your pages incase you decide to add other non-ISO-8859-1 characters. To do this you’ll need to save your files using Unicode-compatible editor (Notepad for Windows will do this if you select ‘Save As’ and choose your encoding). You’ll need to also change your meta elements to say UTF-8 and possibly adjust your server if it’s already serving files as ISO-8859-1. Worth it in the long run though :)