A tour might be a good idea, but to me, it sacrifices some of the adventure factor.
No directions are needed much, because the roads to most of the places require no turns. I did ask ¿Esta es la careterra a Penas Blancas? (is this the road to Penas Blancas?) and stuff like that. Or ¿donde está la calle/carretera a Jinotega?
It only gets a little trickier when you start having to ask where to turn. They will say stuff like "4 blocks up, take a left, go 2 blocks, take a right." That was only in the cities (Granada, Managua, leon, Chinandega was probably the worst).
There are actually signs, contrary to some reports. The road to Peñas Blancas and Jinotega is marked well. If you are going to selva negra, take the Jinotega road that is in Matagalpa, not the one before it. I highly recommend just stopping by Jinotega.
I still recommend taking a car, regardless of your spanish skills. The reports of terrible roads are not entirely true. They are not paved, or smooth, and there are stretches with potholes, but not rutted or muddy, and they are well drained, and are passable even in rainy weather (know this first hand). There are maybe 1 or 2 sections where you might bottom out, and where you will probably need 4wd (by sections, I mean a 25 foot part where there is a rut, or a hole). The terios shouldn't have any problem. If you have the means, a Hilux or Prado would be the way to go, as you wouldn't have to worry at all about bottoming out. We were in a Grand Vitara, and it is by no means capable off road, and had no problems at all, other than a couple bottom scrapes. Not enough for the rental people to notice. Just be aware that if you get a hilux, your credit card supplemental insurance will not cover a truck of any kind.
As for a guide, the proprietor at the lodge got us hooked up within about 10 minutes. No worries there.
So basically, drive through past Matagalpa, take the road to Tuma/La Dalia, dont turn for about 1.5 hours, until you see the giant sign for "Guardianes Del Bosque". Drive down that little road, and you will see the lodge. A little guy with bad feet (poor guy has to walk on his knees, his name is Concepcion) will set you up in the lodge for 5 dollars/night, plus a dollar per meal. He will also get you a guide. Super easy, cheap, safe, fun, and adventurous.
Just remember to pack right.
Edit: I just noticed in my other posts that I said "Northwest" when referring to cloud forest and Jinotega and what not. I meant Northeast, NOT northwest. I always do that. The further east you go, the more it rains. Thats the deal. Damn I write a lot. Hope all of this helps, I'm happy to answer as many questions as you can think of.