Profiling

I advise using RegionScanner to create the json in newer MC versions (1.12+)

Instruction below are only for 1.12 and earlier

You can use /jer_profile [chunk amount] [all] to create a world-gen-scan.json that will get the distributions from a generated world so you don't have to guess. These features are quite advanced and only recommend for pack makers or experienced players. It also requires a game restart to start using newly profiled data.

You can blacklist certain blocks from being saved during scanning by adding them to a file called 'scan-blacklist.txt' in the JER config folder. This file should contain one entry per line and will match the written entries against the blockstate string (see minecraft setblock command). It will do a starting with match, so 'minecraft:' will match any block from minecraft.

By adding world-gen.json to the JER config folder and toggling the DIY setting ingame you can make use of your own ore distributions.