Hey and welcome!
Others will have actual experience to contribute, but FWIW... I haven't started natural dyeing yet but have been flirting for a while. Whether you are dyeing unspin fibre, yarn or fabric doesn't matter too much; but you would use different mordants (substances that help fix the dye to the fibre), depending whether you are dyeing animal fibre (wool etc.) or plant fibre (cotton etc. - quilt fabrics!).
I love, love Jenny Deans book Wild Colour, it explains the dyeing processes very clearly and has information on a lot of plants to dye with. If you get a chance, you might like to have a look at it. It may also help you with figuring out how to use your fungus dye book (it sounds awesome).
Good luck! :-)
Kerstin x