If I understand your question correctly, what I do is I just quote the post and then separate the parts of the full quote, adding 'quote' and 'close quote' tags on those parts I want to separate.
Like this:
Remove the underline character when replying. Just used it here so you could see the code.Code:[_quote=BEN10;874756]I know this might sound silly to someone but I want to know how do I multiquote a post?[_/quote] my answer [_quote=BEN10;874756]I mean if there is a long post which I want to answer using multiquote, how can I do that?[_/quote] my answer [_quote=BEN10;874756]Please tell me every step in that. Thanks[_/quote] my answer
In this example I decided to copy-paste the quote starting tag, which makes it so that every quote bit of text refers to your post. But on large posts, or when feeling lazy, on all remaining quotes past the first one, I just add the quote starting tag without the reference.