Try to cut the tissue so that each piece will have part of the lesion and part of the green or healthy tissue outside the lesion. Usually, the fungus will grow from the edge of the lesion across the nutrient medium. Try to select 4 - 5 pieces about the size of the end of a pencil eraser. Place the prepared pieces onto a plate of nutrient medium using sterile forceps. If possible, use several different media for each kind of fungus.
Some fungi do better on an amended medium.
Try 3 - 4 pieces of tissue on each plate. The fungus will usually grow out and away from the plant tissue after several days on the lab bench top. Use a sterile knife tip or needle to cut a tiny piece of the leading edge of the fungal colony out of the medium and place it on a fresh, new plate of medium. It will be a nearly pure culture of the fungus. After a week or so, you can examine its spores with a microscope.