Hm.. well. So here come some facts.
You need Blender, and strictly BLENDER for this, no other 3D programm would work with what you want.
You -have- to use the Blendfile the Staff uses for making the Meshes, and they are not given out randomly to anyone. Then you will have to learn how to mesh what you want. How to create a Material in Blender.
Then code the Bone Assignment and mesh the Item/Mane/Tail/Ear so it FITS the Bones very exactly and proper.
You will have to know how to do an UV Map for the right Textures of your Mesh.
You need the Right Python Installed.
You need the Ogre Command Line Tools.
You Need the right Ogre Exporter-Script.
You need the Right Program to edit the XML File to be asigned to the right Skeleton.
Inserting custom Meshes to FH will mean you have to replace an existing Mesh, since there are no free Slots.
To say it in one Sentence: It is a lot of Work and recommends knowledge in modelling and programming.
Beginners absolutely cant just do it within a short Time. Even with Tutorials theres no way to learn it proper without a professional or advanced Helper.
I had to go to a 2-Day Course of learning, just to learn some tiny Basics of Blender. And every Course-Day lasted 8 hours. Even after that i had to learn from Kov how some things work.
I dont think someone, who dont has any clue from those Programs, can mannage to do anything :/
But it is welcome if anywhere out there, are people who have and want to help on low poly Meshes.