No player can cast spells or activate abilities in between the modes of a resolving spell. Any abilities that trigger won't be put onto the stack until Collective Resistance is done resolving.

You choose all of your modes at once. You can't wait to perform one mode's actions and then decide to choose more modes.

If an effect allows you to cast Collective Resistance without paying its mana cost, you'll still have to pay the escalate costs if you choose more than one mode.

No matter which combination of modes you choose, you always follow the instructions in the order they are written.

You can't choose any one mode multiple times.

If Collective Resistance is copied, the effect that creates the copy will usually allow you to choose new targets, but you can't choose new modes.

If you chose more than one mode and the target of one of those modes becomes illegal, the other targets will still be affected. If all of the targets become illegal, Collective Resistance won't resolve.