I'm making a journal for my girlfriend and would like to put "from the mind of [name]" on the front page. Sort of a play on "ex libris". If that doesn't really work, or sounds odd, "[name]'s thoughts" or "thoughts from the mind of [name]" or similar would work too. Ex cogitationes?
I would express "mind" here with animus. Lewis and Short list the meanings "rational soul", "senses", "intellect", "mind", and "memory" in their dictionary. Another option would be mens, but I think the somewhat more emotional nuance of animus is a better fit for this purpose.
As ex libris is "from the books", "from the mind" would be ex animo. To make a complete title you would need the Latin genitive of the name or some other word of address. There is no simple "of" in Latin, so forming the genitive is a bit more laborious. Using amica ("female friend"), the title could be ex animo amicae, "from the mind of [a/the] [female] friend".
If her name ends in -a, then to get the genitive you simply replace the -a by -ae. (It is better to think this way than just adding an -e to the end. One of the reasons is that it works differently if the word ends differently.) If, for example, the name is Maria, then you would have ex animo Mariae. If the name ends in something else, you should ask a separate question on this site for help with that specific name.
If you want to use cogitationes, it needs to go in the ablative case: ex cogitationibus. This is certainly possible, but is more "from thoughts" than "from mind".