If you want to get from Penang Sentral Butterworth to Kuala Kedah Jetty, the journey involves having to go via Alor Setar as there are no direct transport options (unless you want to pay a lot of money for a taxi).
The distance from Butterworth to Kuala Kedah Jetty is around 100 km by road.
There are no direct bus services from Butterworth to the Jetty at Kuala Kedah so you will first have to take a bus from Butterworth to Alor Setar Bus Terminal (Shahab Perdana).
From Shahab Perdana Bus Terminal in Alor Setar, you will then have to take a local bus to the Jetty at Kuala Kedah.
Local buses depart from the platform located outside the main terminal building and drop you off right in front of the Kuala Kedah Ferry Terminal car park.
The total trip by bus will take you at least a couple of hours (depending on how long you have to wait for the bus to the jetty) and cost around 12 - 13 RM.
The cheapest way to get from Butterworth to Kuala Kedah Jetty is to first take a cheap KTM Komuter Utara train service to Alor Setar Railway Station.
From Alor Setar station you will then have to walk into the centre of town to take the local bus to the jetty (the walk is around 10 minutes - to find the nearest bus stop it is best to ask for directions from the station).
Travelling this way by commuter train and bus the trip should cost around 10 RM.
The other, much easier but more expensive option is to take a taxi from Alor Setar Train Station to Kuala Kedah Jetty (by road it is 13 km to the jetty from Alor Setar KTM station).