Pretty adorable little animals, guinea pigs! I'm going to attempt to needle felt one for my sister for her birthday.
It's important to start with a good armature, so I search the internet for an image of a guinea pig skeleton, then attempt to copy it with wire (aluminum gauge 14 wire). You can't see it in this photo, but for the feet I made a loop of wire then used cotton-covered florist wire for the toes. Once I was done with the wire, I wrapped the armature with pipe cleaners. The pipe cleaners give something for the wool fiber to stick to-much better than the slippery wire. I like to wrap the pipe cleaners pretty densely, but you really don't have to.
From here it's just a matter of adding wool fiber to the armature. I prefer the core wool that's in a long thick (1 1/2 to 2") rope. I don't use it that thick, but tear off the length I want then split it lengthwise into 2, 3, or 4 pieces, depending on the width I want. For quick form building, I'll use a half, but for toes, I'll use a very thin piece, splitting the 1/4 even further.
In the beginning, I wrap the armature, but once I have a bit of form built up I just place chunks where I want them and needle them in place. (By the way, if you're not familiar with needle felting, one builds form by applying wool fiber over wool fiber. The felting needle has tiny barbs on it, as does the wool fiber. Poking the wool with the special needle interlocks the barbs on the fibers, holding them in place.)
After an hour and a half, this is what I had. I'm more than half through with the core wool. I added the bead eyes (8mm black glass) because, in my experience, once I have the eyes, the creature begins to come to life.
It was a challenge to build from a skinny piece of wire with all the arm and leg joints articulated to a guinea pig shaped blob.