Commission a panel of doctors that are empowered by the federal government to study marijuana and determine once and for all (within a reasonable period of time) if marijuana should continue to be classified as an illegal drug. Making sure this panel is free from political pressure and biased opinions. Are funded well enough to conduct this work and ensure that they are free to express their opinion without reprisals or threat no matter their findings
Then use their opinion to either formulate new (law / policy), change existing (law / policy) or continue with the same (law / policy) regarding the legalization of marijuana. If they determine marijuana can be legalized then the federal government should create another panel to draft guidelines (within 1 year) on the sale and tax of it