Follow these steps to install:

1. Download the zip file to anywhere on your hard drive, do not unzip it (and re-zip if it is automatically unzipped by your browser).
2. Open a new instance of Blender
3. Go to Edit>Preferences. The Blender Preferences window opens.
4. Click the Addons button on the left-hand panel.
5. At the top, click Install.
6. Locate to the download zip file and double click it to install.
7. If the add-on does not immediately appear, look for it under the "Mesh" category.
8. Click the check box to enable the add-on.

To use the Add-On:

1. In 3D View, switch your model to EDIT view.
2. Ensure that the mesh is manifold (i.e. has no loose pieces - this will stall the algorithm that finds a path from the start to end faces - at least each face must be connected to another.). You can use SHIFT-CTRL-ALT-M to highlight any vertices of non-manifold parts. Either delete or join these into the model until it is fully manifold.
3. Clear the face selections.
4. Select the face you want the build modifier to start on.
5. With that face selected (you can pick more than one but only the first will be used) create a vertex group called "SortStart" and assign that face to it.
6. Clear the face selections.
7. Select the face you want the build modifier to end on.
8. With that face selected (you can pick more than one but only the first will be used) create a vertex group called "SortEnd" and assign that face to it.
9. Go back to OBJECT mode and save the blend file to ensure your changes are set.
10.  Go back into EDIT mode for the mesh, and under the Face menu (in the 3D View), select "Sort Mesh by Specified Faces". This can take a while if the mesh is really big.
11. Go back to OBJECT mode and add a Build modifier to the mesh. Try it out. The selection of the start and end faces can make for some interesting effects.