This is a trivial undertaking for anyone who takes more than a few nanonseconds playing with Word.

To turn an entire document into a single paragraph requires nothing more than a Find/Replace, where:
Find = ^p
Replace = ^l (to insert line breaks)
or
Replace = space character (to eliminate even line breaks)
Do note that you're liable to lose a lot of font formatting, etc. if you do either of these, but that is a necessary consequence of doing what you say you want to do.

Double-spacing & justification are easily handled via the 'Paragraph' panel on the Ribbon's Home tab.

That's rather like jumping on a motorcycle for the first time and demanding to be told how to ride it from A to B without at least learning how to use the gears, just because you can do that on that rusty clunker of a bicycle you have sitting in the back of your garage...

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]