No koi keeper should be without a microscope, so maybe it is time to invest in one. They are not expensive.
How to take the scrape:
Catch the koi and place it in a viewing bowl filled with pond water.
Tilt the bowl and let an assistant hold the koi firmly against the side of the bowl above the waterline. (Alternatively place it on a wet towel, hold it firmly and do the scrape. I normally cover the head with the wet towel. It helps to keep it still)
Wait until the koi settles down.
Take the microscope slide by firmly run it along the body in the direction of the scales.
Do it until you can see the mucus on the slide.
Put the fish back into the pond.
Put a drop of water on the mucus and place a cover slip on the mucus on the slide.
Examine the slide for any movement. If you find movement, compare it to the images of the various parasites found on the forum or various internet sites.
If you are not experienced or do not have an assistant, anesthesize koi before doing the scrape. See separate topic under medication.
It is advisable to take scrapes of at least two koi, and choose the ones that show signs of distress.
I often do scrapes with the minimum equipment when I am at client's ponds. On the next photo I am doing it directly from the net without taking the fish out. I found a lot of Trichodenia on this client's koi last week after he complained that his fish are jumping.