Assuming you have already read about the lowlevel libs3d s3d api, the s3d widget library is build on top of it and should be (hopefully) easy to use as any other widget library. Just include
#include <s3dw.h>and link with -ls3dw -ls3d, and everything should be fine. :)
bash$ gcc program.c -ls3dw -ls3d -o program