I usually import both models into the same program and then export both after making sure both models are directly on top of each other, just to be safe. Both models must be in the same exact spot in the 'world,' which is to say their location in any modeling application must be the same. The goal of the first part of the workflow is to achieve a 'Low Poly' model and a 'High Poly' model.

The purpose of retopologizing is to get a low poly model, which is ultimately what will be in game.

If the project is an organic asset like for instance, a character, I start off by sculpting in zBrush first, then after I'm happy with the details, I retopologize the sculpt using zSpheres. That is, after I've set each polygon face to the correct smoothing group before hand, otherwise turbo-smooth will make a mess. If the project is a hard surface asset, I almost always start off with making primitives in 3Ds Max which I then apply a quick turbo-smooth modifier to get a high poly model. Here is the art workflow I developed through painstaking trial and error with the help of online tutorials and lessons at the Art Institutes of Austin.

Material id marmoset toolbag