How to write a negative review for a conference paper? This is related to the general question of "How do I referee a paper?". I am reviewing a paper for a conference, and this paper should be rejected, because it is not significant enough for publication and has flaws in some of its technical details. The paper is not wrong, but the ways in which it is right are not very interesting. How do I write a negative review in such a settings? More importantly, how do I do this as a non-senior researcher?

As an example, they make part of their argument through numerics, but I can prove the same result analytically. The analytic treatment is longer than what I can include in a review. I know the author's emails (the review process is not blind both ways), should I email them? This is a conference, so there is no time for revisions, will they be mad at me for rejecting their paper? Should I email them before or after they receive the reviews?

The paper is also sparsely cited, and does not connect strongly to existing literature. I am familiar with many aspects of the relevant literature, how detailed should I make my recommendations on further reading/references?

To extract the general questions:


How does a junior researcher write a negative review?
What should you do if you have specific technical comments/improvements that are too long for a standard review?
I think this is too much of a duplicate of the other question, but my comment got too long.


Firstly, the review process is blind, so you simply have to do the best that you can. Be objective, be polite, give good reasons – see other question. 
There is probably no such thing has having technical comments/improvements that are too long for the standard review, unless you will end up rewriting the paper. Authors tend to appreciate comments, even if they mean more work. If the paper is borderline and is worth improving, then give as many comments as you have time for.
If you have additional comments that you want to send to the authors, consult with the chair/editor before contacting the authors. Perhaps your comments can be sent anonymously via the chair/editor. 
Papers get rejected, we learn to live with it. The best you can do is provide good comments to help the authors improve the paper for next time round. Authors may get mad, but they don't get even.
If the paper is terrible, just save your time and don't bother with all the comments. Write a sufficiently convincing review giving reasons why the paper should be rejected, point out that there are many typos and other mistakes, but don't enumerate them all.
On the other hand, if you are presenting and proving an alternative axiomatisation of their results to the degree that you are not only rewriting their paper, but lifting their results to a whole new level, then you should consult the chair/editor to see what to do. Maybe you should get credit for your contribution. You certainly should never write up your results independently, not until the paper has appeared in print.