How to make an ordinary site adapted for mobile devices?

Question

HTML

Hello!
The problem is this.
There is a ready-made site, but it is without adaptive design.
How to make it adapted for mobile devices with minimal time costs?
Maybe it's easier to put bootstrap on it or are there any other solutions?
In short, I ask the public to share their thoughts.
comments like,

easier to make a new site

, do not roll, you need to redo the one that is.

Answer 1

Media queries, etc., they’ll write the same) . Download the book: I. Markot, Responsive Web Design, it is small, but at least you will understand how these queries are built and how to make an adaptive layout in general. + There is an example. Read it in 2 hours

Answer 2

Unfortunately, if the design for mobile layout is not initially thought out, the amount of work will be large.
In chrome, in the inspector there is a mode for emulating different devices. Go from a large display to a small one, add the appropriate media query in css for each display width, fixing the layout.
bootstrap will not solve your problem, because. you'll have to redo everything with the bootstrap classes.