I'm not sure how to structure my sentence in a correct manner.
What I want to express is that I've started something, and will let someone know once it (has?) finish.
Here is an example:
I will be monitoring the progress and I will let you know once it is finished.
So I will be watching/monitoring the progress (I will have a look from time to time to see if its finished) and when I see/acknowledge the process is complete I will let you know.