Using MeshMixerMeshmixer's approach to support is different: it uses a branching tree-like structure of well-defined pillars that come to a point right at the location that the support is needed. The diameter and complexity of the shapes can be configured, so that the support can print cleanly, provide a firm base for upper layers, exactly where needed, and then be broken away easily afterwards, with minimal, easily-repaired surface damage.
Optimizing the Supports
Existing supports can be removed by Control-Clicking them (Command Clicking on a Mac), and new supports can be created by clicking on a red supportable area, or existing support, or by dragging from those spots (which gives you a bit more control over where the support line goes, but perhaps not enough to get the perfect support mesh; the line will tend to snap to places that you would rather it didn't go).
The first objective in cleaning up the mesh is to make sure that there is enough support for all the critical areas of overhang. You need to have a support bar at the lowest point of each overhanging area, and if the overhanging part is quite large, and the amount of print to be supported is more than a few mm high before the part will merge into the main structure of the print, then you will probably want to have several separate support beams to ensure that the piece is held firmly as it grows and won't topple under the vibration of the print head moving over it repeatedly.
Next, you want to add extra struts to hold the supports in place - especially where the support beam are long, or leaning over at an angle. Clicking several times on a beam in the same spot will add an array of struts, fanned out around the main beam, giving a wider foot print, and so tends to keep the center of gravity of the strut over its base area, which helps with the part's stability as it grows taller. You can also cross link each island of supports to the adjacent ones to further increase the stability, and to provide some failsafe options incase one of the support struts falls over before it reaches the desired height. If it was cross linked into another support, then there is a good chance that that other support beam can take over its responsibilities higher up in the build, and still provide a base on which the upper part of the support, and particularly the support tip, can be built cleanly. Generally, you will want to work from the center of the object out to the edges, since the outer support struts make it gradually harder and harder to see what is going on in the center. Another handy hint to remember is that the 'c' key works to re-center the view over whatever point the cursor is at. Until you realize that, manipulating the view can be rather frustrating! This picture of the finished Dragon structure shows the sort of complex mesh of supports that can be woven - and the sort of detailed, fragile prints that this tool makes possible.
Export and Slicing
Once you're done tweaking the support structure, you can save the project in a native '.mix' format file that will let you reopen it later, and make further tweaks, or indeed totally remove the support and start over. You can also save a combined mesh file that includes the original object and all the support struts, in several different formats, including .obj, .ply, .dae, .amf and, of course, as an STL file.
You can then import one or other of these into your slicer, and slice it as you see fit (although ideally at the same size, orientation, and layer height as you specified in MeshMixer at the start of the process). And of course, you can turn off any 'automatic support' options in your slicer, as that has all been taken care of for you already.One thing to note is that the various support pieces and struts are not merged together, but exist as separate shells in the finished file. This can confuse some slicers, causing them to leave a hole wherever the bars intersect, and so greatly reducing the strength (and hence usability) of the supports. You may be able to work around this in your slicer - for instance, Cura's 'Combine Everything (Type A)' option in the expert settings makes sure that the overlaps are fully filled in, so that the strength of the supports is unaffected.