You need to do four things.
1) Take the IFP "Instrument - foreign pilot" written test.
2) Take the Commercial pilot written test
3) Take a ME Commercial checkride, based on previous experience. This basically means that with your JAR certificate, you meet all formal requirements and will need only training to proficiency.
You will need (minimum) "3hrs within 60 days" for an instructor to sign you off for the practical test.
4) Take an FAA 1st or 2nd class medical (ok, this you probably should be number 1)