This is something I have been using with my core French classes for 3 years now. Students have told me how much they love it and how helpful it has been throughout the year.
Essentially, when reading a text or during any French class, any time the students hear/read a word they do not know, they will write it down in the French column and follow it with the English translation of the word.
Oftentimes we do this when going through a text as a class and I will write down common unknown words on the board and they will copy them down in their "Dictionnaire Personnel" IF THEY DO NOT KNOW THEM. This helps build their PERSONAL vocabulary. It may be that one students may already know a word while another may not, which is why I place emphasis on only writing the words you do not know so that you become familiar with it and it is added to your personal list of French vocab words you know.
*Sidenote: I usually print several of them, as students fill up their "Dictionnaire Personnel" I hand them another ensuring they filled both sides. I print them on coloured paper so they can quickly identify them in their binders/duotangs.
Hope it helps!