Here's a highly detailed tutorial series on preparing assets for realtime use - including retopologizing, baking normals and additional texturing in Substance Painter.
This is a video tutorial series in which I show the workflow on how to create a game asset from the high poly model to the low poly version.
I hope you like the series, you can download the models and textures for free from my Patreon account.
1. Sculpting the high poly model with Blender
2. Retopology with 3D-Coat
3. Alternative Retopology tool: Blender & Retopoflow
4. Manual UV Unwrapping with Blender
5. Baking normal maps with Blender Cycles
6. Texturing with Substance Painter
7. Export the textures and import to Unity and Unreal Engine 4