Yeah, go right ahead! I know writing it helped me learn a lot about tissues, and the modding community needs all the help it can get at this point.
EDIT: I prettied it up a bit, because damn I cannot write well at 4 AM.
Body parts are defined by the BP tag in the body_default file, and grouped together in collections with the BODY tag, thusly;
[BODY:BASIC_1PARTBODY]
[BP:UB:body:bodies][UPPERBODY][LOWERBODY][CATEGORY:BODY]
[DEFAULT_RELSIZE:2000]
This creates a body part called "body" with a relative size of 2000. Now, if body parts were all just made like that, there would be nothing telling the creature file how to put them together. So for limbs, organs and such, we do it like this:
[BODY:BASIC_3PARTARMS]
[BP:RUA:right upper arm:STP][CONTYPE:UPPERBODY][LIMB][RIGHT][CATEGORY:ARM_UPPER]
[DEFAULT_RELSIZE:200]
...
That CONTYPE tag tells the right upper arm to connect with all body parts that have the UPPERBODY tag. There is no limitation on tags here. If I gave the upper body the SQUEDLEYSPOOCH tag, and then gave everything that connected to it CONTYPE:SQUEDLEYSPOOCH, it would work just fine. If I wanted to connect with that part specifically, I would use the CON tag. The syntax for the con tag is like this:
[BP:RLA:right lower arm:STP][CON:RUA][LIMB][RIGHT][CATEGORY:ARM_LOWER]
[DEFAULT_RELSIZE:200]
The CON:RUA tag tells the body part to connect with the part that has this pattern
[BP:RUA:blah blah blah:blah blah blah]
As you can see, the right upper arm fits this pattern, and so the two parts are connected.
In the creature_whatever file, these collections of body parts are then called via the BODY tag, like so;
[BODY:BODY_WITH_HEAD_FLAG:HEART:GUTS:BRAIN:MOUTH]
Here, things start getting complicated. In the material_template_default file, all of the physical properties of a given material are defined. Ignition point, shear yield, that kinda stuff. Here's the file for skin, as an example
[spoiler][MATERIAL_TEMPLATE:SKIN_TEMPLATE]
[STATE_COLOR:ALL_SOLID:GRAY]
[STATE_NAME:ALL_SOLID:skin]
[STATE_ADJ:ALL_SOLID:skin]
[STATE_COLOR:LIQUID:GRAY]
[STATE_NAME:LIQUID:n/a]
[STATE_ADJ:LIQUID:n/a]
[STATE_COLOR:GAS:GRAY]
[STATE_NAME:GAS:n/a]
[STATE_ADJ:GAS:n/a]
[DISPLAY_COLOR:7:0:0]
[MATERIAL_VALUE:1]
[SPEC_HEAT:4181]
[IGNITE_POINT:10508]
[MELTING_POINT:NONE]
[BOILING_POINT:NONE]
[HEATDAM_POINT:10250]
[COLDDAM_POINT:9900]
[MAT_FIXED_TEMP:NONE]
[SOLID_DENSITY:1000]
[liQUID_DENSITY:NONE]
[MOLAR_MASS:NONE]
[IMPACT_YIELD:10000]
[IMPACT_FRACTURE:10000]
[IMPACT_ELASTICITY:50000]
[COMPRESSIVE_YIELD:10000]
[COMPRESSIVE_FRACTURE:10000]
[COMPRESSIVE_ELASTICITY:50000]
[TENSILE_YIELD:10000]
[TENSILE_FRACTURE:10000]
[TENSILE_ELASTICITY:50000]
[TORSION_YIELD:10000]
[TORSION_FRACTURE:10000]
[TORSION_ELASTICITY:50000]
[SHEAR_YIELD:20000] used data for human skin
[SHEAR_FRACTURE:20000]
[SHEAR_ELASTICITY:50000]
[BENDING_YIELD:10000]
[BENDING_FRACTURE:10000]
[BENDING_ELASTICITY:50000]
[MAX_EDGE:0]
[ABSORPTION:100]
This lets the game know to use this item to determine the exact material type for the leather product in the tanning reaction.
[MATERIAL_REACTION_PRODUCT:TAN_MAT:LOCAL_CREATURE_MAT:LEATHER]
This lets the game know that an animal was likely killed in the production of this item.
[IMPLIES_ANIMAL_KILL]
[ROTS]
Completely independent of this, the tissue_template_default file defines all the biological aspects of a tissue, like this;
[TISSUE_TEMPLATE:SKIN_TEMPLATE]
[TISSUE_NAME:skin:NP]
[SCARS]
[TISSUE_MATERIAL:LOCAL_CREATURE_MAT:SKIN]
[RELATIVE_THICKNESS:1]
Lower is faster.
[HEALING_RATE:100]
[VASCULAR:1]
[PAIN_RECEPTORS:5]
[CONNECTS]
[TISSUE_SHAPE:LAYER]
In the b_detail_plan_default file, material and tissue templates come together. First the materials are imported.
[BODY_DETAIL_PLAN:STANDARD_MATERIALS]
[ADD_MATERIAL:SKIN:SKIN_TEMPLATE]
[ADD_MATERIAL:FAT:FAT_TEMPLATE]
[ADD_MATERIAL:MUSCLE:MUSCLE_TEMPLATE]
. . .
Then the tissues.
[BODY_DETAIL_PLAN:STANDARD_TISSUES]
[ADD_TISSUE:SKIN:SKIN_TEMPLATE]
[ADD_TISSUE:FAT:FAT_TEMPLATE]
. . .
Bear in mind that the b_detail_plan_default file only exists to make the creature_whatever files shorter. These things could just as effectively be done there.
Now we get to the meat of this. Adding the tissues to the body parts. This big ugly line here:
[BP_LAYERS:BY_CATEGORY:BODY:ARG3:50:ARG2:5:ARG1:1]
Is basically saying "Take the (currently empty) upper body, and add 50 thickness unit of ARG3, 5 thickness units of ARG2, and 1 thickness unit of ARG1." What are those? That's up to the creature_whatever file to decide. Let's have a look at the dwarf entry in creature_standard:
[BODY_DETAIL_PLAN:VERTEBRATE_TISSUE_LAYERS:SKIN:FAT:MUSCLE:BONE:CARTILAGE]
as you can see, the first thing after VERTEBRATE_TISSUE_LAYERS is SKIN, so the ARG1 from b_detail_plan_default gets translated into skin. The same applies for fat, muscle, etc. So that line ends up reading as "Take (currently empty) upper body, and add 50 thickness units of muscle, 5 thickness units of fat, and 1 thickness unit of skin." And there you have it! Your upper body is defined. The organs within the upper body are defined by the same method. With a little raw digging, you should now be able to make any body part under the sun!