How to create font with variable weight similar to this example

Glyphs
#1

What would be the steps to make a variable font that displays changes in one glyph by one?

09%20PM
09%20PM1166×1266 183 KB

Say:
Weight 50, I modify only Glyph 1
Weight 75, I modify only Glyph 2
Weight 100, I modify only Glyph 3
etc.

similar to this:

#2

I think that is a rather simple variable font that is controlled by JavaScript.