First make sure you have a strong case that you are the original author, then you contact the relevant people (your academic head and/or the editors of the journal that published the plagiarizing paper) with the evidence and enlist their help.
See guide by AIS for victims of plagiarism.
- Get Some Perspective
Dealing with a plagiarist may be stressful and unpleasant but the odds are on your side if you can prove your case. You need to undertake a realistic assessment of the seriousness of the plagiarism and how important redress is to you before proceeding ...
- Establish the Plagiarism
As a first step, you need to prove that you have been plagiarized. Compare in detail the work of the suspected plagiarist with your own and carefully document the evidence.
Before you allege plagiarism, either publicly or privately, be absolutely certain you have a convincing case. This may pose little difficulty when an entire article or substantial chunks of your text have been used unaltered ...
- Document Your Authorship
Your next step is to prove that you are the original author and not the plagiarist! Early drafts and dated messages with correspondents, editors and reviewers are particularly effective in making your case.
Remind yourself that your comparison of one article with another may only tell others that someone committed plagiarism, but it may not clearly show who. Your next step then is to focus on amassing evidence to show that you are the original author ...
- Notify Your Administrative Head
Avoid direct contact with the plagiarist. Meet with your dean and present the evidence that you have been plagiarized. Ask your dean to formally contact the plagiarist’s dean and request a formal investigation or immediate resolution ...
- Notify the Editors
Increase the pressure on the plagiarist by notifying the editors of the publications involved and requesting redress. Submit your evidence and urge the editors to seek an explanation from the alleged plagiarist. Indicate what restorative measures will satisfy you ...
- Be patient!
If circumstances favor you, the case may be resolved quickly, which is not an uncommon outcome. But if the plagiarist denies guilt or the evidence is unclear or disputable, a resolution may take time, if it occurs at all. Academic institutions, for a variety of reasons, are often less than forthcoming with decisions relating to academic personnel. You may need to be patient!
You might also be interested in COPE guidelines for what editors should do when confronted with a plagiarism allegation. Notably these guidelines assume plagiarism is detected. The onus is on you to demonstrate that there is plagiarism.