This excerpt takes you through the process of creating a simple actor(in this case a cube). This tutorial was created and donated by Chris Whitman.

geActorDef *ADef;
geActor *Actor;
geBody *ABody;
geXForm3d Transform; // XForm to be filled with identity matrix
geVec3d V1,V2,V3,V4,V5,V6,V7,V8,N1,N2,N3,N4,N5,N6; // Points and Normals
int BIndex; // Index to the single bone
int MatIndex; // Material (Just white in this case)
geFloat Size=5.0f;

ABody=geBodyCreate(); // Initialize body data

geXForm3d_SetIdentity(&Transform); // Set Identity matrix

// Create the Bone
geBody_AddBone(ABody,GE_BODY_ROOT,"Bone1",&Transform,&BIndex);

// The second parameter to geBody_AddBone is the parent index,
// If no parent is used just specify GE_BODY_ROOT (which is -1)

// Create a white material
geBody_AddMaterial(ABody,NULL,NULL,255.0,255.0,255.0,&MatIndex);

// The second parameter would normally be the filename of the texturemap
// the third would be the name of the alpha map

// Lets set up all the vertices for our cube

geVec3d_Set(&V1,0.0f,0.0f,0.0f);
geVec3d_Set(&V2,Size,0.0f,0.0f);
geVec3d_Set(&V3,Size,Size,0.0f);
geVec3d_Set(&V4,0.0f,Size,0.0f);

geVec3d_Set(&V5,0.0f,0.0f,Size);
geVec3d_Set(&V6,Size,0.0f,Size);
geVec3d_Set(&V7,Size,Size,Size);
geVec3d_Set(&V8,0.0f,Size,Size);

// Set up normals

geVec3d_Set(&N1,0.0f,0.0f,1.0f); // Front
geVec3d_Set(&N2,0.0f,1.0f,0.0f); // Top
geVec3d_Set(&N3,1.0f,0.0f,0.0f); // Right
geVec3d_Set(&N4,0.0f,0.0f,-1.0f); // Back
geVec3d_Set(&N5,0.0f,-1.0f,0.0f); // Bottom
geVec3d_Set(&N6,-1.0f,0.0f,0.0f); // Left

// Now Create the cube front.  I'm not including texture coords seeing
// as they are unnecessary.
// You could add them if you wanted though.

geBody_AddFace(ABody,&V1,&N1,0.0f,0.0f,BIndex,
   &V2,&N1,0.0f,0.0f,BIndex,
   &V3,&N1,0.0f,0.0f,BIndex,
   MatIndex);

geBody_AddFace(ABody,&V1,&N1,0.0f,0.0f,BIndex,
   &V3,&N1,0.0f,0.0f,BIndex,
   &V4,&N1,0.0f,0.0f,BIndex,
   MatIndex);

// Now add the top

geBody_AddFace(ABody,&V5,&N2,0.0f,0.0f,BIndex,
   &V6,&N2,0.0f,0.0f,BIndex,
   &V2,&N2,0.0f,0.0f,BIndex,
   MatIndex);

geBody_AddFace(ABody,&V5,&N2,0.0f,0.0f,BIndex,
   &V2,&N2,0.0f,0.0f,BIndex,
   &V1,&N2,0.0f,0.0f,BIndex,
   MatIndex);

// right

geBody_AddFace(ABody,&V2,&N3,0.0f,0.0f,BIndex,
   &V6,&N3,0.0f,0.0f,BIndex,
   &V7,&N3,0.0f,0.0f,BIndex,
   MatIndex);

geBody_AddFace(ABody,&V2,&N3,0.0f,0.0f,BIndex,
   &V7,&N3,0.0f,0.0f,BIndex,
   &V3,&N3,0.0f,0.0f,BIndex,
   MatIndex);

// back

geBody_AddFace(ABody,&V6,&N4,0.0f,0.0f,BIndex,
   &V5,&N4,0.0f,0.0f,BIndex,
   &V8,&N4,0.0f,0.0f,BIndex,
   MatIndex);

geBody_AddFace(ABody,&V6,&N4,0.0f,0.0f,BIndex,
   &V8,&N4,0.0f,0.0f,BIndex,
   &V7,&N4,0.0f,0.0f,BIndex,
   MatIndex);

// bottom

geBody_AddFace(ABody,&V4,&N5,0.0f,0.0f,BIndex,
   &V2,&N5,0.0f,0.0f,BIndex,
   &V7,&N5,0.0f,0.0f,BIndex,
   MatIndex);

geBody_AddFace(ABody,&V4,&N5,0.0f,0.0f,BIndex,
   &V7,&N5,0.0f,0.0f,BIndex,
   &V8,&N5,0.0f,0.0f,BIndex,
   MatIndex);

// left

geBody_AddFace(ABody,&V5,&N5,0.0f,0.0f,BIndex,
   &V1,&N5,0.0f,0.0f,BIndex,
   &V4,&N5,0.0f,0.0f,BIndex,
   MatIndex);

geBody_AddFace(ABody,&V5,&N5,0.0f,0.0f,BIndex,
   &V4,&N5,0.0f,0.0f,BIndex,
   &V8,&N5,0.0f,0.0f,BIndex,
   MatIndex);

geActor_DefCreate(ADef);
geActor_SetBody(ADef,ABody);
Actor=ge_ActorCreate(ADef);