Start with getting measurements, and information. Are there any fish? The fish change what can be done and in what order. If no fish, then you can clean out the yuck, change out the water, and get info on how to figure out the filter. I’m no expert there, so I’ll leave filters and pump to the ones who do know. But if you do have fish, I’d start my removal of and dead plants, a slow water change, replace old water with de chlorinated water, and you’ll want to put a rush on a filter. You can get ideas for diy filters if you have a pump, or can get one, just get like a milk crate or plastic laundry basket, and line it with polyfill, sheets of it tend to have less random threads getting lost in the out flow. Set that up so the water goes back into the pond, and when it gets to a point where water isn’t moving well, remove the fill and rinse it out in a bucket of pond water that you’ll then dump on your garden or away from the pond.