Once you download Fishing Crates, Copy the .jar file (paste it elsewere) and rename it from ".jar" to ".zip" (you dont need to extract it). 

Open the data file and copy "fishing_stuff"

Next, install KubeJS. pick the latest version from here:
https://www.curseforge.com/minecraft/mc-mods/kubejs

Pick latest on the version you downloaded, whether it be 1.20.1 or 1.21.1

Open your minecraft folder (reminder: install KubeJS otherwise the next step will make no sense)

Follow this directory:
kubejs -> data

Paste "fishing_stuff" into here.

Now that its installed, you can change the loot tables!

Open the file and follow the directory:
loot_tables -> blocks

From here you'll find all the files for all 4 crates, You may need some knowledge on how loot tables work. 

I suggest using a loot table generator to make it easier. 

Heres one I recommend:

https://misode.github.io/loot-table/

(p.s. the gameplay file has the loot table for the crate drops, you can change this to make crates more common. keep in mind this may break balancing if done incorrectly)

WHEN YOU SAVE THE FILE, MAKE SURE IT MATCHES THE NAME OF THE ORIGINAL FILE AND IS IN THE SAME FOLDER. OTHERWISE IT WILL NOT WORK. 

QNA: why kubejs?
Short answer: i suck at mod dev, if youre editing it, you are a modpack dev and you need kubejs anyway. dont like kubejs? use a datapack, its just as complicated as this (aka not at all complicated)