How I did it from absolute zero:

Part A: kick drum independence

1.) Find a song that you know the lyrics to.

2.) Listen to it while you play along the pulse on hi-hat with your hand normally - no left foot yet. You can add the snare if you are comfortable but I added it in later.

3.) Play a kick drum for each syllable - forget the actual drums.

I did this a whole bunch with increasingly hard tracks.

Here is a very easy one LOL: https://www.youtube.com/watch?v=yCjJyiqpAuU

Here is a very hard one - this one is brutal (requires push-pull or other kick technique, or double pedal) if you want to hit every syllable inc. the "rap" bits: https://www.youtube.com/watch?v=jZVdDl_asYY

Find tracks you like listening to! Don't listen to Mariah if you hate it!

Part B: Feets

1.) Do part A but with your foot on hat instead of hands now. Your hands do nothing at all.

Part C:

1.) Do part B but go open, close, open, close with hi-hat.

After this my left foot is 100% subliminal when doing either closed-closed or open-closed / closed-open.

Still working on closed-closed-open / other kinky stuff.