Just one clarification. If you are traveling in business or first class on TG, you should check in at row A and use the dedicated fast-track immigration there. After passing through immigration there will be an escalator to take you down to level 3 at the entrance to the main TG business/first class lounges. If you are traveling in economy, then follow the other advice here.

The security checkpoint is on level 3, the same level as all the lounges.

In addition to the main TG lounge on Concourse D, they also have lounges on Concourse C and Concourse E, so depending on which gate your flight departs from, one of those lounges may be closer and also likely less crowded.