It's easy really. Just open up (as another layer) the fur texture you want in Feral Heart >> Media >> Textures. Once you have opened it up, go to 'Layers' then there should be a drop down box saying 'Normal'. Click the arrow in the drop down box and select 'Multiply'. If it looks a little jaggady then right click the layer and go to where it says 'Transparency' and take off the alpha channel.