How do you know your paper is ready for submission? I am writing a 3 page letter and my advisor basically told me to prepare it by myself and send it for submission. 

I have written papers before, but in most of those cases my advisors helped me a lot in revising the papers. Perhaps my advisor wants me to see how much trouble it is to publish a paper lol

I have revised and re-revised and re-re-revised my paper, but I am never sure whether my paper will get rejected because my logic is not perfectly right or maybe its horribly wrong.

When do you know you are ready to submit your paper?
The truth is that the paper will never be in perfect condition for publication. This is the role of peer review, to evaluate the work from an expert viewpoint. In general, I have seen that after 2 3 weeks and after 4 5 revisions the quality of the paper increases and reach a saturation point where the paper needs an external opinion in order to be improved. If an external opinion is not possible(the paper might be outside a peer domain), then one must forget the paper and to reevaluate it after some time, and then to submit for publication. However, what is more important is the idea and the results from the paper, and not only the presentation form. Also, the validity of the results and the interpretation is very important.