Blaze Flowers will randomly generate only in the Nether in a newly created world with this mod installed. If you come across one, it can be broken with your hand, or a tool, which will result in it dropping usually 1 Blaze Rod, and more rarely up to 4 Blaze Rods in the latest version. If the plant doesn't break, its not yet fully grown, so give it a little longer to mature, however, it may break and return you the seed for you to start again.
After obtaining some Blaze Rods, it is possible to create a growing area anywhere you like and craft Blaze Seeds to make a garden of Blaze Flowers. To plant a Blaze Seed, you MUST make your growing area out of Soul Sand. Bone Seeds will only grow, and can only be planted on Soul Sand. The Soul Sand will not need to be hoe'd.
Blaze Flowers are slow growing, so be a little patient. If you are not a patient person, you can hurry the growing process with Bonemeal.
Here's how to craft a Blaze Seed