I like to use furnace hackle on a 'general' soft hackle wet fly ("spiders" to the British) that I fish when I don't know which bug to imitate. Basically, it's just a body of spun dubbing, a rib of wire turned on in the opposite direction, no more than 3 (preferably 2) turns of furnace hackle. Simple. The only trick is to get the body not too thick - a nice, skinny body will do with just a few guard hairs sticking out randomly.
Here's an old trick for these wet flies: fish them during a hatch, when you'd normally be using a dry fly. This is especially good if you're not sure which bug the trout are taking. A good 'general' wet fly w/ a gold-ribbed hare's ear body & this furnace hackle can do wonders during a hatch of either caddis or mayflies. Sometimes the trout take these as emerging insects, other times I think they look like a crippled, or stillborn, bug. There are several ways to fish a soft hackle wet to a hatch........
You can grease everything - line, leader, fly - & float them up high (as high as a wet fly can be); or you can grease the line & leader & fish the fly just below the surface. This requires you drying the fly occasionally, like a dry fly w/ false casts, as it will become water logged & sink your line. You can also choose to grease the leader & the hackle only, so the body sits in, or just under, the surface film, imitating an emerging insect. There's a lot of ways to fish these flies.
One other trick: if you'll be fishing these flies up top, in or near the surface, tie them on a dry fly hook. (I can hear the intake of breath from the audience now). Seriously, if you know these flies will be used up higher than a wet fly is usually fished, then give yourself every advantage you can & use lighter, finer diameter dry fly hook.
Furnace is a great hackle, which I almost always prefer to regular brown hackle when a fly's recipe calls for brown. To me it just looks a bit more life-like & that is the entire point of our flies.