Before uploading your design, it is best to ensure that the design is watertight.
It is important to check your design for naked edges and non-manifold edges and fix these issues. Most of the time non-manifold errors will stem from shared edges/points. You may be able to overlap areas or thicken points to help avoid this. Naked edges will show where there is a gap in model.
If there are multiple elements to the design that needed to be printed at the same time and together these parts must be booleaned. You will need to check that they are not just grouped but unioned together as one piece.
You should export the model to a high resolution stl for the best print quality. We would recommend viewing the stl once exported to ensure you are happy with it.