Right now I can't find a guide for making custom ores, which I know is possible.

It's an entire chunk of comments in the config...

    # Format is:
    # modid:block:meta, deposit size, min Y, max Y, chance to gen in chunk, block to use as sample, [dimension blacklist] Example:
    # actuallyadditions:block_misc:3, 32, 13, 42, 20, actuallyadditions:block_misc:1, [1, -1]
    S:"Custom Ore Entries" <
     >

So you would use actuallyadditions:block_misc:3, 32, 13, 42, 20, actuallyadditions:block_misc:1, [1, -1] if you wanted to add Black Quartz to custom generation.

I see. I didn't see that when I checked the configs earlier, but now I see it. Is deposit size the amount of blocks in the vein? Is it exact? Is the chance a percentage or is it one in every x chunks?

It’s randomized approximate quantity. The chance is percentage. Higher chance = more likely.

Thanks!

What is the percentage out of? Is it the percentage that chunk will have a vein?

Out of a hundred, but there are other factors into the actual occurrence of it generating.