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.