If I’m in the National Guard, do I have to get permission or guidance from my unit to apply to ROTC?

No, you don't. You can sign up for the first two years just like regular classes, and then if you decide to contract for years 3 & 4 you will become a Simultaneous Membership Program Cadet (SMP) and be administratively promoted to E-5 (pay, but no stripes). Your National Guard Commander will be informed and may talk to your PMS about you, but you will not need the National Guard's permission.

When I was recruiting for Navy Officer programs. Navy ROTC programs yes, you would need the recommendation from your units Commanding Officer or OIC.

It would make the move much easier if you do.. make sure you packet is lookung good too