Forging stars is actually very easy
In the soon to be dissolved [Cloud World], Noah, or more aptly Alexander- was forging a Nebula within his origin as the essence of universal laws went on to help him do it!
The cores of the Nebula Rank assassin that delivered himself to him continued to be consumed as fuel, more and more stars being formed within the Nebula. The experts at this rank were particularly numerous, as the requirement to proceed into the next realm was particularly high, and it actually varied among different beings.
It was a known truth that after one had cultivated more than 1000 stars within their Nebula, they could make a breakthrough into the Realm of Black Hole if they were talented enough! The talk of talent was mentioned because some beings achieved more than 1000 stars and still remained in the Nebula Rank for all their lives, death claiming them after a few thousand years.