The first thing you have to do is ignore the Gaian base. It's much easier to create a new bone structure when you're not trying to hang onto the old one. So lower the opacity and draw your anthro base over top of it. Start from the torso and go from there. Legs are going to be two joints - one for the knees and one for the ankles, which as you can see attach to elongated feet.
Get your base shading in. Since Gaian bases are really only two main colors with a dark edge this shouldn't be too hard. Then fill in where you missed pieces of the original base and remove where you had too much. Unless you're doing a repose the head and arms should stay the same.
Notice how the only thing you can see is the legs of the old base. Now ditch the original base and go onto adding highlights.
Again, since Gaian bases are shaded really simply you don't have to do too much. Follow the shading Gaia already did. Leave shadows for the eye sockets and under the knees, to give it the perspective of having bended back legs. For the muzzle, start with a round thing between the eyes, put a bar up between them for the nose, and then connect the cheeks to the side of the face.
Sometimes I add belly fur. That's just a lighter shade, repeat the same shading as before.
Here's some models to follow. For the face, just imagine circles. Then, connect the three main circles together. For legs, think ovals. One for the thigh, one for the leg, one for the foot. The leg is always bent back, the knee bent forwards slightly.
Aaand the finished product! It's a kangaroo rat! Feel free to use this model as a base for you own anthro edits, altough I ask that you attempt your own or make significant changes to mine to make it your own.
