Instructional Designers need to understand how to prioritize and present the material that enables the learners to quickly absorb the material. It requires having at least a basic understanding of the source material and the learner skill level so they can build the course (ILT or SPT or blended) in a way that makes it easy for learners to see the real world application of that material.
The best way to develop your skills is to build courses (ILT, SPT or blended). If you're focus is SPT, then get trail versions of different software and create a simple course or three. It doesn't have to be current work related. It could be as simple, real-life practical things like how to make a sandwich, or preparing to move, or even how to create a post in an Indeed.com forum.