If you have the set representation of the minterms, then it remains to follow the variable ordering for the Shannon decompositions along that variable ordering. For instance, if we have variables c<b<a and the set representation {{}, {b, c}}, we first make a case distinction on variable a. Since a is false in both of the two represented minterms, these define the 1-subtree while the 0-subtree is the false leaf. We continue with a case split on {{}, {b, c}} where we get {{c}} for the 1-subtree and {{}} for the 0-subtree. Finally, {{c}} is the BDD for c, and {{}} is the BDD for !c.
