This is how I did it. It could fit only my design. My pages look like a blog page. The DIV floating left is the main content. Then I follow it with small DIV boxes for ads. The main DIV is 40em wide. And has side margins 4em each. That totals 48em = 768 device pixels.
So, I added the following meta instruction to every page:
<meta content="width=768" name="viewport">
That nicely gets the page on the smartphone, except the text is too small. The ads slide down to the bottom. Next, I went to my CSS file. There, I format my regular text with 'para' class:
.para {font-size:1.2em;line-height:140%;font-family:arial,helvetica,sans-serif;color:#333}
@media only screen and (max-width: 768px) {.para{font-size:2em;}}
The upper line is for PCs and the @media one is for phones. Notice that I increased the text size to 2em for the screen just 768px wide.
This all passes Google Mobile Friendly test. You can view-source my web site:
Read source-code of this and test it here.
NOTE: I keep changing my work. This site might be gone in few days.
Good luck.