methodologies

Object-oriented methodologies are much more recent than C, and are really beyond the language (that is the realm of C++).

Some older methodologies that can be followed in C are structured programming and modular programming.

structured programming

The C language was originally designed to be structured, to follow the structured programming methodologies.

A structure is dumb data that models a conceptual thing. (A Java programmer could think of it as a class without methods or inheritance.) Functions are thought of as operations on such things.

modular programming

A module is a structure with behaviors, which are routines tightly bound to the structure, forming the programming interface to the structure. Its implementation is hidden from the user — one accesses and manipulates internal structures solely through an interface.

Think of a module as a final class, with no inheritance.

While modular programming is a more recent concept than C, the language is powerful enough to facilitate this methodology.

example:
modularized linked list header
modularized linked list source
complete list module header
complete string module header