In the file gui/spawnerMenu.yml, find the line which has Booster-Lore-Line, usually at the bottom. From there, you can edit the line that'll show up in the spawner GUI when the player hovers on the booster part. You can use the placeholder %booster_name% show the booster's name, %booster_multiplier% to show the booster's multiplier and %remaining_time% to show the remaining time of the booster, which will be updated each second.
There are 3 types and those are INCREASE_SPEED,INCREASE_XP and INCREASE_LOOT.
Multiplier
This is the value that will be used to multiplie the current amout by.
Display Name
This is the name that will be displayed for the booster in the spawner GUI
Expiration Time
This is the time that the booster will expire after it's been used. It'll be paused when the player removes the booster and be resumed when the player re-uses the booster. %expiration_time% changes as player uses the booster.
Item
All of those are the parameters for the item that will be given as a booster to your players.
For the materials refere to the Minecraft name of the item. (F3+H)
How to add new boosters ?
You just create a new section like this:
OLD BOOSTER.YML:
speedboost:
type: INCREASE_SPEED
multiplier: 2
display-name: "&b&lSpeed Boost"
item:
material: PAPER
name: "&c&lSpeed Boost"
lore:
- "&7This will increase spawner speed by x2"
xpbooster:
type: INCREASE_XP
multiplier: 2
display-name: "&a&lXP Booster"
item:
material: experience_bottle
name: "&a&lXP Booster"
lore:
- "&7This will increase spawner XP by x2"
lootbooster:
type: INCREASE_LOOT
multiplier: 2
display-name: "&c&lLoot Booster"
item:
material: DIAMOND
name: "&c&lLoot Booster"
lore:
- "&7This will increase spawner loot by x2"