Infected plant material can be pressed, although if there is superficial growth, this will be damaged and may be lost. Most people use gentle warmth for everything from leaf-spots to fleshy toadstools.
This can be as simple as the top of a radiator, although purpose-built driers such as those sold for drying fruit give the best results. Large fleshy species can be placed in the air stream from a fan heater for fast, effective results even with fragile species like inkcaps.
Infected leaves can be placed in folded blotting paper, lightly weighted on top to prevent them curling up too much.
The time taken to dry varies with the method, but can be from a few hours for infected leaves, overnight for small to medium toadstools, or longer for large toadstools or brackets. Large toadstools can become sealed by a dry layer on the outside, but leaving them at room temperature for a day allows the remaining moisture to soften this skin so drying can be resumed
Leaves will curl and go crisp otherwise weight is the easiest way to tell when a specimen is dry. At this point it's best to leave them at room temperature for a day to soften up, otherwise they can be very fragile.
Dried material keeps reasonably well but is attacked by a variety of pests, especially booklice and mites, and to a lesser extent museum/carpet beetles. Dried material is conventionally stored in paper envelopes, but this gives no protection from insects, so the envelopes need to be stored in batches in sealable plastic bags or boxes. Avoid storing the specimens directly in individual plastic bags as static electricity from handling the bags will make extracting the dried material all but impossible.
Except for some of the more distinctive larger species, microscopic examination is always required to identify fungi. Staining is often necessary to make hyaline tissues and spore ornamentation visible - the question is what stain to use: as a rule of thumb: if it has basidia, use Phloxine; if it has asci, use Melzer's Iodine; other hyaline ascomycete structures can be stained with Congo Red. Anamorphic fungi generally stain well in Cotton Blue, although dematiaceous hyphomycetes often need no staining.
Photographing spores can be difficult, especially under oil immersion when they move continuously.
Basidiomycete spores should always be examined in a spore print, and if the print is made directly onto glass, the spores often adhere (presumably via the Buller drop) and many even remain in position with Melzer's Iodine. A variant on this method is to deposit the spores onto the coverslip, so the plage faces upwards.
Various thickening agents can be used for spores which won't stay put, Glycerine Jelly is best but can make some spores collapse. Methyl Cellulose (aka wallpaper paste) is often used for similar purposes. Xantham Gum (on the "Free From" shelf at the supermarket), Alginate or Gelatine could also be tried.
Both methyl cellulose and xanthan gum contain structures that show up under contrast enhancement techniques (eg Phase Contrast, Differential Interference Contrast): bamboo ropes for methyl cellulose and minute granules for xanthan gum. (Either of these may vary with the source of the material).
Both Gelatin and Methyl Cellulose react strongly with iodine. Xanthan Gum reacts only very weakly.
Xanthan Gum can be prepared by adding the powder to water at about 1:5. Stir until all the white trapped air has disappeared. It keeps for at least a few weeks, but seal to keep out dust and prevent evaporation. To use touch the surface with a glass rod (or finger tip!) and touch onto the slide. As the coverslip is pressed down it will form a thin film without too many air bubbles.