I am fairly new to the Simpl programming but I have my interfaces built first but don't assign joins. Then I move over to Simpl and start creating the logic and through the logic my joins are created, which then get assigned to my VTPro project.

I may not be doing this as recommended/efficiently but find that it works the best for me as it allows me to work in sections and ensures me that my joins are assigned properly, and my program compiles cleanly.