Where does the comma go after or before which?

The short answer: you don't need one.

The long answer:

Which is a subordinating conjunction, meaning that when it starts a clause, it makes a dependent clause which is an incomplete sentence.

Grammar dictates the following comma placement in the pairing of clauses:

(Independent Clause = IC, Dependent Clause = DC) IC DC or DC, IC

Since which will start a dependent clause, provided that you put the independent clause first, you need not place a comma on behalf of the Word which. If, however, the dependent clause goes first, you must place a comma after the entire clause, not just the Word which.