If you have a specific student whom you are thinking in your mind.
1. You stand nearby his/her desk as if it didn't matter.
2. You don't see him/her. You don't make eye contact.
3. Depending on the condition, you can place your hand on his/her desk naturally. (not always)
4. You should make eye contact with the whole class.
5. You proceed with your class lesson at the place.
6. In the case of after making some instructions to let them do something, you can tap his/her shoulder casually, or you can say something a little to him/her. (not always)