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It's ok to ask the editor (before accepting) to co-revise it with your advisor. This is a good way to start and editors are likely to agree on the deal.
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Your job is to summarize (i) the main contribution of the paper (ii) any major concern you have with methods/interpretation of results, etc... (iii) minor concerns and in line minor comments.
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Your job is not to accept or reject the paper. Your job is to suggest how to improve the paper.
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If you are not familiar with any method and can not evaluate it's soundness, its ok. Just explain this to the editor. E.g. I can't comment on method X as I am not qualified to assess it's soundness. This is ok.
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Be very polite and think about it as a conversation with the authors.
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Trust your knowledge