new here - Pool completely GREEN - Not closed for winter

HELP - my pool was not closed for the winter due to illness. My son has been getting the leaves out daily. He believes he has them all out, I will check tomorrow.

We have a 27,000 gallon pool. Hayward DE filter (sorry I do not know the model # right now, but can get it tomorrow and add to the thread). I need to know where to start. We have spent over $800 in the past trying to get it going when t wasn't closed. PLEASE HELP us not do this. I read the BBB, and can definitely do this. Where do I need to start? I know I will need a LOT of DE, because of cleaning the grids, filter, and a lot of backwashing from the algae. I do have an aquachem testing kit.
It smells like a stagnant pond. Any advice is appreciated.

Thank you so much!