Find BibTeX on ACM Digital Library Database
Step one: Click the Export Citation
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Step Two: Select BibTeX Style on the box and copy the citation result. 
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Step Three: Paste the result to the Overleaf. (Instructions of using Overleaf for writing Research paper)
Find BibTeX on Google Scholar
Step One: Click "Cite" on the article result.
Step Two: Select the BibTeX on the left bottom of the cite box.
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Step Three: Copy the result from the new site. 
Step Four: Paste the result to the Overleaf. (Instructions of using Overleaf for writing Research paper)