Download Skriptand DecentHolograms with the links above.
Add main.sk and kills.sk to your plugins/Skript/scripts directory. (recommended you create a folder there and store it there)
Go back into Minecraft and create a hologram. Edit the top 3 lines to your liking, and then add 10 more blank lines below that.
Enter the kills.sk file (never edit the main.sk file) and setup all the config. (THIS DOESN'T NEED TO BE ONLY A KILLS LEADERBOARD, I'M JUST USING IT AS AN EXAMPLE)
Reload the Skript and your leaderboard should be set!
If you are fine with 1 leaderboard, you can enjoy it and skip the rest of the documentation. If you want to have more leaderboards continue to follow the documentation.
Exit out of the kills.sk Skript and duplicate it's file and give it a new name of the leaderboard you want.
MAKE SURE to config all the config options to your liking in the new file.
Create a new leaderboard, following the previous instructions.