I just spent ten minutes figuring out whether or not it is correct to use an apostrophe for "how to's." My gut said no; but I found out that the rule is, if it's a verb or some other word that isn't normally used in the plural, you DO add the apostrophe. "Do" is a verb, although actually in modern usage people use the plural of "do" all the time. We all have multitudes of "dos" to do! (This rule apparently also allows for people to use the possessive apostrophe for decades, like "the 70's" since a decade doesn't really have a plural state. But I personally will NEVER use the possessive apostrophe for decades. I'm digressing, I know.)
Welcome to Brownie How To's!