First of all, I am not an expert on becoming an Instructional Systems Designer (ISD). But, I have been asked by a few people and I would like to present some of the resources that I usually refer people to.
If you want to become an ISD and you have never worked in the field before, you will need to develop your portfolio of projects so that you can prove to your future potential employee that you have the skills. To do this I would suggest starting with an internship. The internship will more than likely be unpaid. 🙁