[OJFord]

Singular: guru; singular possessive: guru's; plural: gurus; plural possessive: gurus'.

(If the word ends with an 's' you can (or not) add only an apostrophe and omit the added 's', afaik it's down to style/how you'd pronounce it. 'Socrates' work' for example, because 'Socrates's' looks and sounds awkward.)

[Minor49er]

Exactly this. Also, the other points in the list already follow this: "Research your keywords", "Do not pay for SEO courses", etc.