Variant 1: You roll initiative, highest roller goes first.
Variant 2: Whoever declared the first combat action goes first. There is no roll for initiative, ever.
From there the variants are the same.
The top roller takes their turn. Then, that person chooses who goes next. It may be a friend or foe.
That second person goes. Then, they choose who goes next - they cannot choose someone who has acted already in the round.
The chosen third person goes. They choose who goes next - they cannot choose someone who has alraedy acted...
Lather, rinse, repeat until everyone has gone. Initiative refreshes.
Whoever went last in the first round, chooses who goes first in the next round.
Lather, rinse, repeat.
Note, there can be some strategy here - the party will be tempted to take all their actions first. But then the villain(s) go, and can choose themselves to go again first in the second round. If you don't polish them off before you act, they can act twice in a row without you being able to intervene, which can be nasty. PCs will often want to insert the opponents in the middle of the order to avoid this.
When using larger groups of opponents, I will tend to group them: Big Bad, Melee Minions, Ranged Minions, for example, as three groups the Players can give initiative to.