I think this was the procedure about a year ago.
Nothing to National B - pass medical, pass ARDS test
Nat. B to Nat. A - get 6 signatures from club level events, 1 of which can be for marshalling.
Nat. A to Int. D - the holder of a National A can upgrade to International D without any further criteria, just the cost of the upgrade (the difference between the two plus an upgrade fee)
Nat. A or Int. D to Int. C. - get Signatures from: 3 National A races OR 2 National ‘A’ plus 2 National ‘B’ or Clubman Races OR 6 National ‘B’ or Clubman Races. All from three different venues.
Int C to Int B and Int. B to Int. A. - compete satisfactorily in at least 10 national races or international races within the previous 2 years, and if you've finished in the top 5 of those races and finished within the top 5 of a suitable important championship, then you can get Int. A rather than Int. B.
Int. A to FIA Super Licence - accumulate at least 40 qualifying points within three years. Winning GP2, Indycar, F3 European series gives you 40 points. Winning national F4 gives you 12 points.