Hi gang,
Next week I have an appointment to re-new my long gun permit. Never did it before. Anything special I need to fill out in advance before I go to the office?
Also, my permit expires July 24, 2020. So I need to show up 14 days prior (on July 10th) to apply for renewal, am I correct?
Thanks in advance for any help! 
You don't need to show up 2 weeks before expiration. That's the EARLIEST HPD will accept a renewal application, so you don't have 2 valid permits simultaneously.
Theoretically, if you show up exactly 14 days before the LG permit expires, you can pick up the new one and not have a lapse. Unless you plan to buy a LG close to the expiration date, you will still be able to renew the permit after the expiration date with only the normal 14 day delay in buying & registering.
It's the same process for renewal as it is for a new LG permit.
The only thing extra is HPD requires you to return the existing permit.
So, if you have a LG permit, take it with you both when you apply and again when you pick up the new one. If it's already expired when you renew, they will keep it regardless.
If you can't find your LG permit, HPD normally requires you to sign a statement explaining why.
When you say "show up", I assume you know there are no walk-ins accepted now, right?