Assuming you only want it for you own games , the best thing you can do is to create your own ruleset for that (so your changes won't be lost upon game or mod updates).
At the minimum you will need an extraStrings.rul file under your mod folder (you can name it anything you want) and ensure it is loaded after all the mods for which you want to override the texts, in practice it means put it at the bottom of your modlist ingame.
Since you are mentioning FMP it is probably easiest to use that extraStrings.rul file, eg copy
mods/Final Mod Pack/Ruleset/extraStrings_FMP.rul -> mods/mySuperDuperExtraStrings.rul
and adapt that one to your liking, you only need to keep the entries you want to redefine.
Vanilla descriptions can be found at
bin/common/Language/en-US.yml
(in the program folder).
Since there are some texts that follow a specific syntax (not sure if it applies to your case) it is smart to use the
translation guidelines .
If you want to change more than just strings, take a look at the
ruleset reference (this links opens at the extratext paragraph).
Have fun
P.s. To the experienced modders .. please correct me if i stated something erroneous.