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Once you have your surface data imported and configured to appear as you wish, you can start building your drainage system.
Keep in mind that the goal is to build a computer model representation of what has been built in reality, or what will exist in reality, depending on the type of plan you are working with. You may have to make some assumptions during the model build process, but the goal is to create a model that is as close to reality as possible.
Best practice is to place junctions, then connections, then stormwater objects, then inflows. Connect drainage system objects starting upstream and working your way downstream.