What is the best way to learn music theory on your own?

Music theory is a vast topic. Wikipedia is the best place to learn music theory as per my experience. Having someone who can help guide you on what to learn next is a better option if you are starting new.

A lot of music theory is based on mathematics. As there are lots of scales and all scales can be played in 12 different keys, things start to become more complex. However, the patterns are consistent, which makes it learnable.

I found the Music Theory Companion Android app very helpful for understanding scales, chord progressions within scales, and the Circle of Fifths. It also has a built-in metronome and allows you to play chords alongside it, so you are always connecting theory to sound.

Whether you use an app, books, or online resources, the key is consistency — study a little every day, and connect everything you learn back to your instrument.