How can I perform loop closure in graph SLAM ?

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Hi, I am trying to implement graph based SLAM and I am using g2o for backend. So far I have

1. Constructed a graph using laser data i.e pose-pose graph
2. I dont have any odometry source so I used Scan_matcher to estimate pose constraints

Now I dont understand how to add and loop closures in my graph. I read some papers but the implementation details are naive to me. I would be thankful if someone could point me in the right direction Thanks