After importing, rotating and scaling the model, I copied it to a new file to work with it further, while saving the model in the old file. To make sure to not change the model in any way, after pressing N and selecting the 3D object, I clicked the lock symbols next to the scale.
First step in cleaning the model is changing from object mode to edit mode. This can be done by changing the mode button in the top left corner or using the shortcut Tab. Now the object will be displayed by showing all the vertices. This can be changed next to the mode button to see either vertices, edges or faces (Image 1). For cleaning the model I found it to be the easiest with the vertex select setting. To get rid of the big areas of vertices, which can be deleted without destroying the object, I chose box select on the top part of the toolbar on the lefthand side or by pressing W. To be able to select all vertices, I sometimes changed the shading to wireframe but most of the time I used viewport shading but also clicked the x-Ray button. This makes selecting and deleting faster, but also lets you make more mistakes, because sometimes I selected and deleted parts of the model, I should not delete. So careful with the x-Ray toggle! I used the top view (Z or Numpad7), selected areas that could be deleted, which then show up in yellow (Image 2), so I could check if I didn’t select to much again… To delete the vertices, I first went to Mesh –> Delete –> Vertices and then used shortcut X. I repeated the process, in important areas, I used the lasso select. On thing that helped, was selecting the areas from inside the model. This way I could use the x-Ray and not select everything on the other side and delete those parts accidentally. But even so, I always checked the model for holes! (Image 3)
At the end I selected all (shortcut A or Select –> All) then went to Mesh –> Clean Up –> Delete Loose, to clean up my model and delete unnecessary vertices, edges and faces (Image 4). And as always: Save!