Hymm. theres different ways to do it because there different kinds of THC derivatives.
I know that one method for synthesizing (-)-delta1-Tetrahydrocannabinol can be done in a simple one step process by reacting p-mentha-2,8-dien-1-ol with olivetol giving about a 50% yield. Theres diferent ways it can be done, I think most of them involve the adddition of two compounds together. one of the addition reactants seem to be compounds with 1,3-hydroxysubstituted benzene rings with a organic substituent in the 5 position on the aromatic ring. They are more or less special reagents, so they are probably very hard to come by.