Step-by-step guide

Edit March 6, 2019: Used SolidWorks 2018 for this guide.

Definitions

Let's start with a few definitions. 

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Let's start with some simple models in SolidWorks to create molds out of. The method we are using is to create the negative space around the part. This would include intersecting the part with a "block" and removing the intersected space. 

Simple Mold (with a symmetrical shape) - TO BE EDITED

You can use any simple shape (sphere, rectangular prism, block, etc.) but for the purposes of this section, we will be using a sphere. (Please see insert link for tutorial page here for instructions on how to create these shapes)

To create a mold from the intersection between two (or more) shapes, you will have to have the shape (see above) and create a separate part that is a solid block. Notably, this block should be larger than the cross-section of the part in which a mold will be made from it.

Once you have these two parts, create a new assembly file in SolidWorks and add the two parts. You can use the perspectives and drag/drop for the objects to intersect (INSERT INSTRUCTIONS FOR INTERSECTION). Note, if the part is larger than the mold block, don't worry. That can be adjusted later when the intersection is removed. 

Select the body (solid/extruded block): Go to Assembly > Edit Component then Insert > Features > Cavity to remove the intersecting part.

Next, set the other object (the object being molded) to invisible to view the mold.

When exiting, remember to rebuild to apply the changes.

Now, when you open the block's file, there will be the indentation, or space, deleted through the Cavity function. If the mold's size was not large enough to accommodate the part, you can edit the size of the mold in its respective file.

TBA: Screenshots

Complex Mold (with a more complex part) - TBA

Example of