I will help you now, if you will help me tomorrow. Is it correct/possible?

does that make sense, according to 'one author'?

If you help me now, I will see you later. (help first, see later). correct.

I will see you now, if you will help me later (see first, help later). Is it correct?

If you helped me now, I would see you later. correct.

I would see you now, if you would help me later. (same as before: see first, help later). Is it possible?

Thank you all.

Including the first (optional) will in constructions like If you [will] do this, I will do that is primarily an indicator of willingness, rather than a future tense marker.

Usually you can only include that first will if it would make sense to use [BE] willing [infinitive] instead of will [tensed verb]. Failure to observe this "rule" is common among native speakers of some other languages, so it's something of a shibboleth indicating "non-native Anglophone"...

1: If you will help me [then] I will pay you
is okay because
If you are willing to help...
is a reasonable alternative.

2: If you will cry [then] I will dry your tears
is NOT okay because
If you are willing to cry...
isn't a natural thing to say.

In principle, using would instead of will in such contexts amplifies the "hypothetical" aspect of the possible future action. But in practice it's usually just a conventional marker of deference / politeness.

As OP has correctly noted, it's often okay to use Simple Past If you helped me I would pay you for the IF-clause (in which case the THEN-clause must be explicitly marked as "hypothetical", using would not will). Note that this Simple Past usage is more likely where the IF-clause represents an alternative to the current situation, rather than something that might happen in the future, but this isn't always the case. For example,...

If you helped them tomorrow, they would pay you next week

...is perfectly idiomatic to many if not most native speakers.