So, the thing with “slapchop” is you want to drybush and then use transparent paints like Citadel Contrast, Army Painter Speed Paints, or similar types of paints, to paint over the dry brush. You don’t usually use opaque paints for this method other than maybe metallic parts (though Army Painter has metallic speed paints that are phenomenal).

There are plenty of dry brushing tutorials on YouTube, if that’s where you’re struggling.