Don’t get me wrong, I think there’s a place for how-to’s (afterall, I do write and consult them myself!), but it shouldn’t be your only step when you tackle a new subject. Trust in your painting knowledge and ability, allow yourself time to try (and fail), to experiment, then take a look at what other people have done with a similar subject, and combine that knowledge with your own, hands-on experience.
This is what I said in reply to the email: "Mmm, swamp scenes ... the only book I've got with anything specifically on this in the index is "The Big Book of Painting Nature in Watercolour" and then it's got only four pages out of 400 on painting moss. But there are also pages on painting foggy/misty scenes.
"However, instead of spending your time trying to find the perfect "how-to" instructions, why not spend your time trying to get there on your own, using the abilities and knowledge you have? Gather some reference photos (searching in Google's image tool will get you loads; just remember, you’re using them as reference, not copying them exactly, which would infringe the photographer’s copyright), then study them deciding what it is that you want to capture (is it the mood, the fine detail, the dark dampness, etc), then do a series of small experiments focusing on mastering one element of the scene at a time, and then in a few weeks' time put it together in one larger painting.
"For moss, for example, take a toothbrush or battered brush and flick some paint onto the ground (paint a base layer in a darker green and flick a lighter on on top). For mist, use experiment with dry-brush effects (load a little paint on a dry brush, then wipe most of it off until it seems the brush is dry, then 'scratch' this across the painting). For mood, be bold and paint the scene a lot darker than you think it should be. Or paint over the whole thing with a dark glaze (I find Payne's grey great for making a too-bright scene moody).
"Don’t be intimidated by a challenge. You may not achieve what you see in your mind’s eye the first time you try that subject (or even the second, third, fourth time), but every painting will take you a step closer. No-one expects to be able to play the piano without learning their scales and practising them regularly. You’re not wasting paint, nor canvas, nor time. You’re doing the artistic equivalent of practising your scales."