Instructional design is also called learning design or didactic design. It starts with the question: What should the participant be able to do afterwards? Then you choose content, sequence, exercises, and tests.
A well-known model is ADDIE: analysis, design, development, implementation, and evaluation. You do not have to follow it slavishly, but it is a reminder that a course should be tested and improved after it has launched.
You do not need to be a trained instructional designer to build a good course. With clear learning objectives, short pages, and a quiz that checks what matters most, you can get far.