Establish the standards consumable product must meet - legislate the active ingredients  (THC, CBD etc) to qualify for recreational and medical cannabis - with regard to other ingredients, toxins and particulate leave that to the be determined by the users and require the seller certify, warrant and guarantee in order to be "recreational" or 'medical" legal cannabis.  Set the government portion of pricing as a function of cost of production and assure the users a paying for the product it's use and abuse consequences.  Then require the licensed producers to monitor and regulate the production of cannabis - a supply management for cannabis.  

The most pressing concern that needs to be addressed before 'legalization starts and is complete' is  with regard to identifying and full recovering the cost of cannabis consumption will impose on exiting medical, health, transportation and education practices in each province - but perhaps that will be addressed by the provincial government.