Either pairs with or. Keep the two halves parallel, and make the verb agree with the closer subject. Either the timer or the flags are on. The negative pair is neither nor.
Match the parts
Either… or… should join the same kind of thing. Either to guess or skipping is not parallel. Do not mix either with nor. Whether introduces a different two-way choice, in if versus whether.
Closer-subject agreement is in subject-verb agreement. Parallel lists are in parallel structure.
The bottom line
Either takes or. Same shape on both sides. Verb follows the nearer subject.
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