There is usually install instructions in the snippets. Some times you need to copy certain files to your ftp, and sometimes just copy paste code to manager..
1.How to install all those modules,snippets etc. in your download section.
Look at the documentation, especially here. Also snippets usually have some parameters what you can use in the snippet call, they are usually documented in the snippet (if not on resource repository, look inside the archive or at the top of snippet code).
2.When i create new document from admin panel how can i include available snippetss in doc.