3D scanners are useful, but a lot of times they don't produce a perfect 3D model. Usually you will need to clean up the scanned file in order to do anything useful with it, like making a 3D print.
Blender is a powerful free 3D tool that can be used to fix and edit 3D scans.
I won't try to fully teach you how to use Blender here. My aim is to demonstrate some of the tools in Blender so you can get an idea of the workflow and functionality. Primarily, I'm using the Shrinkwrap Modifier and the Sculpt Mode feature. There are other software packages with similar features, although they are not free.
The video shows the main steps. You can spend much longer time than 10 minutes on the modeling and sculpting parts and make a really nicely cleaned up model with far fewer polys, but the basics do only need 10 minutes. I also wrote out the steps so it's easy to scan through.
Unfortunately my video doesn't have any audio since I don't have a microphone. I will update later if I can fix this.
Step 1: Import 3D Scan Into Blender and Fix Orientation
The 3D scan file I am using is a scan of a vintage shoe last that I made using a NextEngine 3D scanner. The NextEngine scanner is quite high resolution, and their native software does a decent job of fixing major artifacts and reducing the poly count. I exported an stl file from the NextEngine software.
You can see that the scanned model is pretty good, but it has some holes in the mesh and the front toe part didn't scan well.
Open Blender and import the STL.
The file will likely be in mm units, so you will need to zoom out your viewport a lot.
Change your viewport to "Orthographic View".
Rotate the model so it is oriented the right way.
Step 2: Model a Rough Solid Shape Around Your Scanned Object.
You don't need to be too precise. I started with a cube and used Extrude to form a blob along the axis of the scanned file. Then I used Sculpt mode and the Grab brush.
Make sure there are no parts of the scanned model that poke out.
If you spend more time to model this shape, you can get better results, but as you can see it works okay with something modeled in a few minutes.
Step 3: Shrinkwrap the Rough Shape Around the Scanned Mesh.
Add a Subdivision Modifier to your blob mesh. I went with 3 subdivision levels.
Add the Shrinkwrap Modifier to your blob mesh and choose the scanned mesh as the target.
Now you have a mesh that has the same shape as your scan, but without the holes.
Step 4: Use Sculpt Mode to Clean Up Your New Mesh.
Use the Smooth and Grab brushes in Sculpt Mode to smooth out any jagged areas. I fixed the tip of the toe this way.
Step 5: Export Your New Mesh.
You can export your new fixed mesh into a variety of formats from Blender. Or you can make more changes.