At that level there's way more to it than coming top of the league, because there is a pretty big gap between Southern League and everything below. Teams at step 9 (AKA step 5 of non-league) have to apply for promotion at the start of the season. That triggers a round of tests by the FA, mainly to make sure that the club has facilities which meet the requirement of the level above. And that they have the appropriate financial/governance, that sort of stuff. Pass those tests, and finish in the top 3 of your league (usually), and you go up.
So, in the Wessex League, 3 teams applied, and as Moneyfields FC passed the tests and finished 2nd, they got promoted to the Southern League. Some mysterious committee has to sort out all the geography.