I have been trying to do this recently! I'm already familiar with the repertoire, so I'm going into more detail, but you can start with broader strokes.
I started here, and went by era:
https://en.wikipedia.org/wiki/List_of_classical_music_composers_by_era#Overview
You have 5 eras: Renaissance, Baroque, Classical, Romantic, and Modern.
You can neglect the Renaissance era for now. For Baroque, Classical, Romantic and Modern, start with the most famous composers. I would mostly start in order, although you could flip around if one of them starts to get boring.
For Baroque, with keyboard music, you have Bach, Scarlatti and Handel.
Look up all of the composers in big font, starting from the Classical period onward:
For each of those composers, look up their most famous works. You could go to YouTube and sort by popularity. I also find "Best of" compilations on YouTube of composers to be a fairly good representation, albeit slightly generic and pandering to popular tastes. Use this to compile a small list of pieces.
For each composer, I would pick one piece per style for a survey. For instance, if the composer is Chopin, you might look at one Chopin Ballade, one Chopin waltz, one Chopin mazurka, one Chopin concerto. There may be many more good ones, but that will quickly get overwhelming, and you are looking to understand the idiom.
There is also something of a continuous line starting from the 1600s to the 1900s, and you will observe the music gradually grow in complexity and build upon previous ideas. Having a basic understanding of harmony, and especially functional harmony, will allow you to see these patterns even better as you go through scores.
Good luck.