Instructional designers can use Wiggin and McTighe's Understanding by Design (UbD), van Merrienboer and Kirchner's Complex Learning (CL), or something else. Designers can make up their own method, pulling ideas from various sources.
Skill helps course creators implement their course designs. For example, UbD has the concept of "big ideas:"
An idea is “big” if it helps us make sense of lots of confusing experiences and seemingly isolated facts... For example: “the water cycle” is a big idea for connecting seemingly discrete and one-way events (the water seems to just disappear as it evaporates).
Skilling has "principles." You can use them to implement UbD's big ideas. You create principles as separate learning objects, and drop them into lessons as required. That helps you reuse principles in different lessons. Skilling also creates a principle catalog for students. It's a list of principles used in a course, with links to lessons where principles appear.
The key point: Skilling helps course creators implement UbD designs.
Complex learning (CL) is an alternative to UbD, though they have much in common. CL emphasizes whole tasks, that:
...are authentic whole-task experiences based on real life tasks that aim at the integration of skills, knowledge, and attitudes
Kirschner and van Merriënboer
Skilling's exercises can be used for whole tasks. Like principles, exercises are separate learning objects that are dropped into lessons as desired. Exercises have rubrics, used by Skilling's formative feedback system.
The key point: Skilling helps course creators implement CL designs.
You can use UbD, CL, a different method, or mix-and-match to create your own approach. Skilling has many of the elements you'll find in the learning design literature. Skilling can be extended, as well. You can add your own elements.