I'll give you an example using my beloved 108th FIFA ranked Finland.
Hopefully based on UEFA ranking we end up in the D division of Nations League (placed 40th or below) http://www.footballseeding.com/national-ranking-uefa/ranking-2017/
Teams in D get drawn into 4 groups of 4. Groups are drawn in November. Finland get Macedonia, Luxembourg & San Marino.
Games are played in the autumn of 2018. This is also when the regular qualifying groups for EUROs are drawn.
Finland end up winning their group in the Nations League. Other group winners are Faroe Islands, Armenia & Moldova.
Finland doesn't make the top 2 in their normal qualifying group.
In March 2020 the Nations League playoffs are played among group winners. Although Moldova is replaced by a group 2nd Belarus after Moldova shocked everyone and earned a spot through the normal qualifying system.
Finland beat Armenia in the semi and Belarus in the final. Euro 2020 spot in the bag.