This is universal workflow for any of typical CAD software or typical CAE preprocessing software. Generally speaking there are always a few steps to do one by one. Note: this particular example workflow is given for an existing CAD model to be pre-processed for CFD simulations:

1. Load – Load or import the model into the CAD software or CFD preprocessing software.
2. Explode – Complex model needs to be exploded (split) into the smallest parts.
3. Clean – Remove or hide all those parts that take no effect on fluid flow and all those who are not needed for the CFD simulation. This process is called cleaning. After the model is cleaned up. Usually, just parts which are in a direct contact with a fluid remain visible. NOTE: It is useful to remove all those parts of the model, that take no effect on the fluid flow. It makes the further work more transparent and easier.
4. Close – The surface model shall be made closed waterproof. All the holes or cracks need to be closed.
5. Simplify – Some complex model parts can be simplified. This step is a little bit tricky and needs some experience. This step is also very dependent on case complexity, also on results expectations, on which physical phenomena is about to catch, etc. etc. … – e.g.: – tiny parts, where no effect is expected in comparison with the size of the main parts of the surface model. They can be simplified or replaced or mucked. – negligible complexities of the model. They can be simplified or replaced or mucked. – consider what parts you want to really mesh and simulate. Simplify all the rest.
6. Merge – Merge corresponding parts of the model into groups, which correspond to the designed model boundaries (patches). This merging process creates your CFD model boundaries – inlets, outlets, walls, interfaces, symmetries, cyclic boundaries, etc, etc … It is reasonable to thing twice here – remember that all your boundaries will need their boundary conditions for every simulated quantity. And also remember that many boundaries are always better than just a few.
7. Refine – This is for advanced users only. Check the surface model refinement (resolution) for your STL model export. The surface model refinement shall be so fine to represent the real model in a sufficient way. Some CAD systems do this only automatically with no options available. Some other CAD systems allow a user to mesh the surface model manually which might be useful.
8. Export – Export your STL files. It is highly recommended to export each surface model part (boundary in CFD simulation) into its individual STL file. For example the OpenFOAM meshing tool snappyHexMesh captures the edges much better from individual files, than from a single block.