Installation:

1/ Download the .zip addon file 

2/ Then open Blender and from the menu, go to "Edit -> Preferences -> "Add-ons" -> "Install"

3/ Metaballify addon will appear on the list of addons, search it and activate it.

Location of the the interface: 3d viewport > Object mode > Metaballify 

Get started

1/ To start a session of Metaballify, press the "Launch" button.

2/ Select the meshes that you want to convert, and press the " convert to M-Objects" button to apply.

You can add a new mesh at any time during a session, but you will have to press this button again to properly apply conversion.

3/ If you want, you can add a tweakable objects. Press Add M-Egg for example.

They are 3 Types of tweakable objects, The M-Egg, the M-Cone and the M-Flex.

When you click into M-Egg or M-Cone, two sliders appears in the panel of the addon. You can stretch and taper this objects.

The M-Flex Object can be stretch when you displace the spheres at the extremity.  

4/ With the viewport section, you can filter what you want in the view. 

5/ Into global section settings, you can tweak some aspects of the global shape. When the density slider is set low, the fluidity can improve, but artifacts ( holes ) can appear. You have to search the good balance. A Value of 20 is good way to start.

Into this section, you can set the smooth shade of the mesh. The "Smooth it" setting is good for polishing your shape

6/ In Varishape section, you can see presets that affect the shape of the global mesh. Try them to see wich adapt to your workflow, or the desired shapes.

7/ To close a session and convert the result to a new mesh, press the " Convert to mesh " button.

8/ The "Clean" button delete the loose parts of the new mesh ( into the mesh and outside )