How do I transfer from NGB to Army Reserves?

The process requires you to obtain a conditional release from the Guard to join another component, a DA 368.
With this "license" in hand, you need to find a Reserve unit with an appropriate vacancy or willingness to train you. As a 2LT, your best bet is to find a unit that has an appropriate slot but is willing to train you to be what you're looking for. That way you continue to develop competency in your basic branch in the event things change.
Once the gaining unit accepts you, the Guard must actually discharge you.
After this, you do a new Oath of Office (DA 71) and access into the USAR.

What are your goals, sir?

Team,

It's not a difficult process, but each state will have a checklist for completing the transition. Your S1 can contact higher for the most up to date checklist as more than likely it has changed. The first step is to contact the Army Reserve Careers Division at the link below. They will assign you an Army Reserve Career Counselor for where you live and want to transfer to. It’s an easier process if you are enlisted, but my response if for officers. You can either transfer to the IRR for the reserves to pick you up, or you can transfer straight into the reserves. If you are using Tricare, then the only option to not lose your coverage is to transfer directly into the reserves. If the guard screws up your paperwork like mine, you can contact the NG liaison at HRC (Career Manager [login to see] ) and they can turn it around in 48 hours and backdate all of your orders. Then you’ll have to contact Tricare to make sure everything updated correctly. The Career Counselor will fill out the majority of the information for you once you provide what they need, but below is a list of documents that will be submitted for the process.
DD368 – Request for Conditional Release
Memo to BN Commander
DA71 – Oath of Office
DA 4651 - Request for Reserve Component Assignment or Attachment
A document stating that you have read AR600-20, Chp 4, para 4-23 (Federal Recognition Board: Statement of Compliance)
Once everything is completed you will have a completed DD 368 with signatures, NGB 0123E, Separation Orders and NGB 22. If you are transferring straight into the Reserves, then make sure that they put your gaining unit in block 9 instead of the control group. The control group is the IRR.
If you are in North Carolina, Tennessee, Kentucky, or South Carolina you can contact SFC Katrina McNeil directly. She was tremendous help.
SFC Katrina H. McNeil
4th BN, Army Reserve Careers Division
500 Wilshire Ave SW
Concord, NC 28025
Office: [login to see]
Mobile: [login to see]
Fax: [login to see]
Email: [login to see]

http://www.usar.army.mil/Commands/US-Army-Reserve/ARCD/ARCD-Resources/

Best bet, talk to your Career Counselor/Retention Office. Or, your S1 and maybe even the S3 shops.