When choosing between all ready or already, you have to remember that while they look alike, they have very different meanings.
There is only a difference of two characters when you consider whether to write all ready or already.
But those two characters allow for a significant difference in meaning. The one-word option and two-word option are not interchangeable.
The phrase all ready refers to a state of being thoroughly prepared for something.
The students spent the evening studying and are all ready for the exam.
The phrase can also be used to refer to a single person rather than a group, though the all sounds a bit awkward:
With that heavy jacket, you look like you’re all ready for cold weather!
The sentence in that case might well be just as effective by dropping the all, but as something of a colloquialism, the meaning is generally easily understood.
Already is an adverb and it refers to time.
In one respect, it can refer to something that has happened in the past, and sometimes earlier than expected:
Do you mean John already left with the deliveries?
It can also express frustration about a future event that may or may not have happened?
You’re already preparing for the 2020 budget?
For someone who is exasperated about something that has not yet happened or something they wish had happened before the present moment, it can even become part of an interjection:
And I’ll take that as a cue to wrap up this post!