The bus from Penang to Kangar, capitol of Perlis cost RM7.60. There is no bus service in Penang island, so you need to take the ferry, cross to Butterworth to get the service.
Then upon arrival at Kangar, you can either take a bus or a taxi to Kaki Bukit. The taxi driver will wait for full car load to start the journay and it cost RM2 from Kangar to KB.
Kaki Bukit is half an hour's drive from Kangar, the capital of Perlis.
Wang Kelian is the border town around 12 KM from Kaki Bukit. You can shop for Duty-Free items at the Duty-Free Shop and also various Thai goodies.
You can walk across without a passport! But be careful not to stray too far !
The Perlis State Park is located at Wang Kelian.