Both models should be in a T-pose. They are auto-scaled to the same height and stood on the floor when loaded.
Export each Photoshop layer as its own PNG with transparency. They all get baked through the same transfer map, so they stay in register. Where the source has one texture per material, set each layer to that material.
Untick fur, hair, fluff or other shell geometry that sits just outside the skin. Rays hit those layers first, and because they live in a different UV sheet the bake ends up alternating between unrelated images — that is what mottling and stray colours are. Untextured helper materials are unticked automatically.
Both models are already rigged, and a rig is anatomy somebody has already labelled. Joints present in both skeletons are matched by name — across naming conventions, so Left_wrist finds Hand_L — and the source is bent through them. That puts muzzle, eyes, ears and hands roughly in place, which is what a nearest-surface search cannot work out on its own.
This happens when you press Run fit, immediately before the shape is deformed. Loading a model only scales it to a common height, so the two sit apart until then. The sliders and the button below are for touching up by hand; the log reports how many joints matched.
Pair up points that mean the same thing on both bodies — nose tip, ear tips, fingertips, toe tips, mouth corners, tail tip. The fit only knows what surface is nearest; landmarks are what tell it what surface corresponds. A dozen well-placed pairs keeps a texture anatomically lined up and stops parts with no counterpart from being dragged onto their neighbours.
High stiffness moves the whole body together; low stiffness lets small areas settle onto detail. It ramps from start to end across the iterations, as does the search radius — wide at first so parts held at different angles can find each other, narrow at the end for precision.
If a limb or tail is left behind or hangs outside the target, raise Search start until it is at least as large as the distance that part is out of place. That is the setting that lets it swing across. The log reports how much of each model went unmatched.
Ray distance is how far to search along the surface normal before falling back to the nearest point. Raise it if the two shapes differ a lot.
Files are saved as retex_<original name>.png at the chosen resolution, in the target model's UV layout.