This tutorial will run through the basics of editing a pre existing item within 3D Studio Max for use within Besiege... as i have no knowledge with blender.
You can download a tutorial for 3D studio max .Here:
http://www.autodesk.co.uk/products/3d...
This video does not cover re-skinning the object
Dont forget to use the settings to enable skins in game.
Sorry for the coughing :/