Common Custom Block

When you make the block, the last page mentions 'Block spawning/generation properties'. Set the dimension to generate to Overworld, or your dimension if you want it to spawn in there. Set the block to replace grass blocks, leave the biome restriction empty. Set the ore groups per chunk to relatively high if you want it common, and ores per group can be whatever you want. Set the minimal generation height to 62 (This is Minecraft's sea level), and the maximal generation height to something like 100, or whatever you deem best fits.