if you have the credit card sized BTS ticket proceed to the metallic barriers. Choose one of the ones with a big green tick showing. This indicates it's an entry point. The Red X's show exit points coming from the other side.
Place your ticket into the lower slot on the barrier. The ticket will be taken in and pop out on top as the red barrier pulls back. Take the ticket back and quickly walk through.
At the back of the BTS ticket is a small map showing you all the BTS stations. Helpful if you forget the direction you are going.