Instructions:

If you're playing with third-party mods that add ore, first disable ore spawning in the mod's config files.

1) Disable ore spawning from mods (in the mod's config files)
2) Create a Minecraft test world for your setup
3) Write down the ore block IDs you want to generate according to your custom rules
4) Open the file config/custom_ores.json (created the first time you launch the game with this mod)
5) Copy the ore template and add it below

Field descriptions:

comment: Field descriptions (optional when copying)
name: Your ore name, so you can navigate the file
block_id: Block ID. You can find it by entering the Minecraft world and pressing F3+H. Hover your mouse over the desired block in your inventory or the JEI menu.
block_meta: In some mods, ores share a block ID, but their meta_data differs. For example, in the Mekanism mod, ore blocks share a common block ID of mekanism:oreblock, but if you look at the block description in JEI, they will have 1000/0, 1000/1, and so on. Thus, copper ore will have a meta_data of 1. Specify the meta_data number of the required ore for the mod to work correctly.
use_custom_target: If this field is True, the selected block will be generated in the block specified in the replace_block_id field. If False, the block will only be generated in stone blocks.
replace_block_id: Block in which ore will be generated if use_custom_target = true
min_y: Minimum height for ore generation
max_y: Maximum height for ore generation
vein_size: Size of a single vein
veins_per_chunk: Number of veins per chunk
spawn_chance: Chance of generating a single vein 1.0 = 100%, 0.02 = 2%
dimension_ids: List of dimension IDs in which ore can be generated. Separate multiple dimensions with commas. [0,1,2...]

Recommendations:

When you have already disabled ores from other mods and are starting to create spawn rules for new ore.
After making changes to the custom_ores.json file, save it by pressing ctrl+s in the file itself.
Next, return to the open Minecraft world and run the /ore reload command.
This will update the config cache, and when loading new chunks, the new generation will be used, eliminating the need to re-enter Minecraft each time. Only the server administrator can edit this file or use the /ore reload command.

Ore Assistant