The principle is simple: Take the eight FIFA windows over a period of 14 months UEFA would use to hold qualifying for the European Championship or World Cup, hold qualifying for the last five windows and use the first three windows for the Nations League group stage.
The 2018-19 Nations League ...
-- UEFA's 55 members were divided into four leagues, based on their UEFA national team coefficients after the conclusion of 2018 World Cup qualifying, and further divided into four groups.
-- The teams will play four or six group games (depending on the size of the group) over the six dates available during the September, October and November windows.
-- The four group winners in the elite League A will advance to a final four to be played June 5-9, 2019. (Because that window also serves as one of the five windows for Euro 2020 qualifying, the four finalists will be drawn in available Euro 2020 qualifying groups of five, allowing them to use their two byes in June.)
The competition also includes two other aspects:
Pro/rel. There will be promotion for the group winners in Leagues B-D and relegation for the last-place group teams in Leagues A-C.
Euro playoffs. The carrot to hold out for participation in the lower leagues is that the results of the Nations League will be used to feed the teams that enter European Championship playoffs. Each of the four leagues will hold four finals in March 2020, and the four winners will complete the 24-team field at Euro 2020.
The new format eliminates at least two and up to all the slots used for friendly games over the eight FIFA windows. UEFA's philosophy is to organize more meaningful games and more games between teams of an equal standing.
Money-maker. There is a financial aspect to this, of course. A new league organized by UEFA means more games for broadcasters to air and more games for sponsors to get their message out. Since UEFA controls the broadcast and commercial rights to qualifiers (even World Cup qualifying) as well as the finals, that's significant.
UEFA's revenues from the sale to broadcast and commercial rights to its national team competitions will double from 1 billion euros ($1.16 billion) for 2014-18 to 2 billion euros ($2.32 billion) for 2018-2022. In the last cycle, more than 95 percent of the revenues will have been distributed back to UEFA's 55 federations.