The information below is extracted from the FA Handbook, where more detail can be found – http://www.thefa.com/-/media/files/thefaportal/governance-docs/rules-of-the-association/2019-20/national-league-system-regulations-2019-2020.ashx
The Clubs in the bottom two places in each of the two divisions at Step 2 at the end of the Regular Season will be relegated to a feeder pool and placed in the most geographically appropriate division at Step 3 for the following Playing Season. They will be replaced by the Clubs finishing in 1st position in each of the four divisions at Step 3 together with a further four Clubs determined by a series of Play Off Matches. Where a Club finishes in 1st position but does not meet the criteria for participation at the next Step, the Club finishing in 2nd position shall be promoted and the Club finishing in 3rd position (and so on) shall be the Club in the next eligible position to take part in the Play Off Matches. Where a Club finishes in a Play Off Position but does not meet the Criteria for participation in Play Off Matches the Club finishing in the next eligible position shall take part in the Play Off Matches. In each division at Step 3, the Play Off Matches shall be played so that the highest placed of the eligible Clubs plays against the lowest placed and the other two Clubs play each other. The ties will be played on a single match basis with the home Club being the Club with the best points per match ratio in the Regular Season. If there are only three eligible Clubs then the highest placed shall receive a bye to a second match where it will play the winner of the other Play Off Match and the winner of that match will play in the further Play Off Match described above. Clubs finishing below position 7 will not be considered for Play Off Matches.
The promoted Clubs will be placed in a feeder pool and placed in the most geographically appropriate division at Step 2.
The Clubs finishing in the bottom two places in each of the four divisions at Step 3 at the end of the Regular Season, along with the two third from bottom Clubs with the lowest points per game at the end of the Regular Season will be relegated to a feeder pool and placed in the most geographically appropriate division at Step 4 for the following Playing Season.