Postals are just like assignments. You will have to submit one for each subject. First of all marks obtained in postal papers are not of any relevance i.e. they wont be considered at any stage, this is just for clearing postal papers only.
You will find most of the answers in the module of the institute itself. In case of any problem you can take help from b.com books, guideline/suggested answers of ICSI, or you can refer postal papers submitted by anyone else you know of same series. You can also post your specific queries at this site for suggestions and answers.
Try to avoid copy-paste technique. After referring answer try to rewrite it in your own words and make full attempt. And dont take postal papers too seriously, concentrate on your studies with reference to your exams. However postal papers can give you hint about the contents of subject and type of questions that can be asked.