My answer focuses on the header question about decades—which is the question that most readers will probably expect to find answers to here. With regard to decades expressed in numerals rather than spelled out in letters, some style guides recommend omitting an apostrophe, while others recommend including it. For example, from The Chicago Manual of Style, fifteenth edition (2003):
9.37 Decades. Decades are either spelled out (as long as the century is clear) and lowercased or expressed in numerals. No apostrophe appears between the year and the s.
the nineties
the 1980s and 1990s (or, less formally, the 1980s and '90s)
The Associated Press Stylebook (2002) adopts a rule very similar to Chicago's:
decades Use Arabic figures to indicate decades of history. Use an apostrophe to indicate numerals that are left out; show plural by adding the letter s: the 1890s, the '90s, the Gay '90s, the 1920s, the mid-1930s.
The Oxford Guide to Style (2002), while differing with Chicago and AP on capitalizing the spelled-out decade name (Oxford University Press prefers this) and on using abbreviations like '60s (OUP condemns this), agrees with Chicago that the plural numeral form should be spelled without an apostrophe before the s:
To denote simple ten-year spans OUP style prefers, for example, 1920s or 1960s to nineteen-twenties or nineteen-sixties. To denote decades of a specific character (say, the Roaring Twenties, the Swinging Sixties) OUP prefers Twenties or Sixties to '20s or '60s.
But Words into Type, third edition (1974) takes the opposing view:
In referring to decades, the sixties or the 1960's is generally preferred (not '60's, '60s, 60's, or 60s; the last form is used occasionally for ages of persons).
The New York Times Manual of Style and Usage (1999) agrees with Words into Type about the apostrophe, although about little else:
decades should usually be given in numerals: the 1990's; the mid-1970's; the 90's. But when a decade begins a sentence it must be spelled out. [example omitted]; often that is reason enough to recast the sentence.
Clearly the question of how to render a particular decade in print is a style issue on which reasonable style guides may differ. Follow the one you have to follow, or choose the one you like.
Part of my answer to the question posted here was:
According to ‘The Cambridge Guide to English Usage’, the removal of
the apostrophe from 'plural nouns in phrases which express affiliation
. . . is widespread in the English-speaking world' and has the
imprimatur of the American Associated Press stylebook and the
Australian government Style Manual. As the Cambridge Guide says, ‘the
time spent worrying about whether it should really be driver’s licence
or drivers’ licence would be better used elsewhere.'
Nothing is lost by writing kids books rather than kids’ books. However, the Cambridge Guide does point out 'that there are special cases which seem anomalous without the apostrophe s', and one of them is childrens, 'because that is not a regular form of the word . . . Thus context is the final arbiter as to whether apostrophes are needed, as always.'
Best Answer
They are both correct. The use of an apostrophe to form a plural of a mentioned term (which is what you would be doing here) is valid, though less commonly used than it used to be. (It's one of a few cases where apostrophes are, or have been, used to form plurals, on which you can read more here).
It is now most often seen in set phrases like "if's and but's" or "do's and don'ts" (note that the double apostrophe of don't's is normally avoided) because some people who no longer use the form do still with set phrases.
But meanwhile, the trend is increasingly against this form.
It does though have the advantage of clarity as you say, so it is arguably the superior if you have no formatting ability, or in handwriting.
If you can use formatting, then the likes of:
Can be better than either; being even clearer that you are using "How To" as a name for a how-to guide (so it is a noun phrase rather than an adverb-preposition pair, and hence a sort of mention) and pluralising, without the rather old-fashioned use of the apostrophe in a plural.