Then you create a JSON file in the compostables folder, this can be named anything but I recommend naming it something sensible.

My JSON file named 'composting' looks like this:
{
    "values": {
       "minecraft:diamond": 0.65,
       "minecraft:gold_ingot": 0.86,
       "minecraft:iron_ingot": 0.23
 },
    "can_villager_compost": false
}

Values takes in an item and a float, the float is the composting chance.
Can villager compost is a Boolean and when true the villager will be able to compost any items in 'values'

I recommend making two JSON files in the compostables folder, one for items the villager and the player can compost and one for items that only the player can compost