You ready an action as if you took the ready action,however the readied action can triggered any time during the duration of the spell.In addition should the action be triggered,you can choose to perform the action as if you were in the location where you cast this spell. 

at higher levels

for every two levels beyond 2nd that the spell slot used in this spell is cast using, you can ready an additional action using them.When you trigger a reaction  readied with this spell, you can perform the action from the location the spell was cast or any location in which you performed other reactions with this same spell.