What is some constructive advice or guidance that will help me prepare for an S1 position in the National Guard BEB?

Definitely subscribe to the S1Net email

Apply? will this be an AGR position or just MDAY UVP move?
Is your BEB S1 position AG branch or EN?
I have some more specific guidance pending those answers, but here is the general info:
Your BN S1 shop is primarily a proof-reading step for S1 and MED actions going up to 130th MEB BDE S1 for action, there are very few personnel actions that complete at BN level.
Your S1 shop should already have an AGR PSNCO and MED NCO (ideally both).
And your BN AO (AGR MAJ, ideally the BN XO) is the chief of staff that directs the fulltime workflow. If your job is MDAY, you should talk (email) with these people once a week.
If your job is AGR, daily.
You already understand the Evaluations Entry System, you need to start using the Evaluations Reporting System (found on the EES homepage).
You need access to iPERMS, specifically to see Records Review reports and results.
You will need access to SIBx if NC ARNG is using that system. If they use SIBx, let me know, I have a detailed what and why for that. And it is the most useful S1 tool in the Guard.
You will need Record Brief and DPRO access, (DPRO especially if your state does not use SIBx.) Download and peruse the user manual for Record Brief, in the Guard there is precious little we can manually update on the Record Brief, and people will ask you about the data that we cannot manually update.
Get on the Army Publishing Directorate and download the most common 600 series regs and pams: Evaluations, awards, promotions, flags, etc. And then get on the NC ARNG G1 site and download any NC ARNG reg or pam they have available.
The NC ARNG G1 site should also provide checklists and even template documents for every personnel action, download any checklists so that you can understand the process for each from BN thru BDE to G1. (Each state has their own tweaks to these).

You will be the SME for officer actions within your BN, but you also have to understand (and brief) the enlisted actions, and what causes the delays for both and how to correct them. If you don't have a MEDO and/or PA, you will also have to understand and brief MED/MND.

When it comes time to counsel and rate the NCOs in your S1 section; most of what they do is in a system that can easily generate a quantifiable number, (total documents indexed or reviewed in iPERMS, total enlisted promotion packets and enlisted advancements, total flag actions, total MND cases completed, total awards processed, to total transfers/ISTs/separations processed, as well as error rates for all), use that on their NCOERs.

Start Googling for S-1 SOP and learn.