Kind Latin geeks! How do I decline a proper name?

I have a bunch of bookplates. They say “Ex Libris _____”, and you’re supposed to write in your name. I’d like this to be somewhat correct, so how do I correctly decline my first name (“Nicole”)? Is “Ex Libris Nicolae” correct?

There are five declensions in Latin. The fourth and fifth are not used for names, so let’s ignore those.

The first declension goes (in the singular)

Nominative Amanda
Genitive   Amandae
Dative     Amandae
Accusative Amandam
Ablative   Amanda

So the -ae ending is from the first declension, and you could use that. One problem, though: first declension words always end with a in the base form, which your name does not. Of course, you could fudge it, and pretend:

Nominative Nicole
Genitive   Nicolae
Dative     Nicolae
Accusative Nicolam
Ablative   Nicola

But let’s look at your other options.

The second declension is only for male names, so let’s look at the third, which is gender-neutral. It goes like this:

Nominative Iuno     Iuppiter
Genitive   Iunonis  Iovis
Dative     Iunoni   Iovi
Accusative Iunonem  Iovem
Ablative   Iunone   Iove

It may seem like the first form doesn’t have much to do with the others, and because it doesn’t: you can’t immediately tell what the stem of a3rd decension word is from the base form, you need the second form as well to know how to decline it.
Which makes the 3rd decl. perfect for making Latin names out of non-Latin names. You just need to pick a stem. If you choose Nicol-, you get

Nominative Nicole
Genitive   Nicolis
Dative     Nicoli
Accusative Nicolem
Ablative   Nicole

I’d Latinise your name Nicole to Nicola (since women’s names generally end with an a in Latin) and then use the genitive Nicolae. So ex libris Nicolae.

Just Say No.