You will need an .XMODEL_EXPORT file to start with. If you do not know how to export your animation to .XMODEL_EXPORT , check out the page BO3 | Export Models/Anims from Maya 8.5+
BO3 has a new compiled format for xmodels called xmodel_bun. All of your exported xmodels must be converted to this format in order to be loaded by Asset Property Editor (A.P.E.).
In order to convert the files, you must use export2bin.exe.
Once you have your .XMODEL_BIN files, you are ready to set up the xmodel in Asset Property Editor (A.P.E.). Start by creating a new asset in your GDT of choice.
For the Asset Name, enter a name for the xmodel you wish to use. I often recommend using the same name as the xmodel file although it is personal preference.
For the Asset Type, choose "xmodel".
Then click OK to create the xmodel asset.
Once you create your xmodel asset, you will see a confirmation screen with lots of options, most of which you can safely ignore.
For "Type" you must select a type for the model. If this model will be animated, choose animated - otherwise the correct option is rigid in nearly all cases.
Next you need to set the "File" under the "LOD 0" section to your XMODEL_BIN file you created earlier. Once you set this, you should see a preview of your model in the preview window, but it will not have any textures.
If you scroll down to the Materials section of your xmodel asset, you will see a list of all the materials the xmodel needs. The ones which are missing have a yellow exclamation icon next to them. Simply click the icon to create the materials you need for your xmodel. For more information on creating materials and textures, see BO3 | Adding custom materials/textures.