In MN 61, The Buddha instructs us to regard mental acts that we have done That led to harm for ourselves, others, or both with a particular trio of perceptions, horror to prepare the mental space and hold the perception of the problem, repulsion towards the problem action for the sake of strengthening self restraint in the mental area, and disgust to lubricate the operation and push it to completion.
First off horror leaves you vulnerable to thievery more so than normal, so only do this where you’re unlikely to be stolen from, psychically speaking. or in emergencies. This is because you are in a self induced semi-fright state where you grab hold of the perception of the thing being regarded as horrifying, but let go of mental objects to the periphiry, actually holding them out like some Tim Burton horror jump scare. Further it can be used only so far as a mental object that is effectively made of tough imagination can be stressed without breakdown. They’re hard to build, these perception grabbers through horror. Start out trying to put out the minimum horror you can, and learn how to be horrified gently. Just a slight opening of the eye, etc.
Repulsion fails through overuse by going crazy for me, I just flail at the problem uselessly. Or if it gets behind my defenses, repulsion fails through throwing bits of myself at the problem instead of repelling it. Do not let that be you, learn small amounts of repulsion and measured targeting with the horror. Put your repulsion where it is needed most. I have been imagining the perception of innate control of force fields, and this seems to be serving me well. but I think the perception also needs to “remember” to be repulsed by what it is repelling. What I’m trying to say is that there is a quality to repulsion that is emotional. And I think the quality seems important. Repulsion can be tuned to purpose, use words like “get that ________(propensity to act out of ill will {for instance}) out of me!” where you describe the repulsive action. When mental objects are too close, it is difficult to keep the repulsion as repulsion, it wavers into grabbing on to prevent further incursion, which can then be twisted into attraction by an evil mental object, so do your best to catch the problems before they encroach too far. When they do get in that far, the only thing I can tell you is that the bhrama viharas can exhibit field effects and equanimity gives you the best chance for unwaveringly repulsing.
Disgust fails through running out of disgust juice then devolving into ill will, then burning you. Beware. Go gentle and influence your (disgust generating mental object) with the bhrama viharas often. As before, I recommend experimenting with small amounts as best you can. This seems to make the perception sandwich easier to swallow. My go to way to perceive disgust is at the need for the whole thing or the damage done to innocent mental objects that got in the way. Disgust can be preserved and protected by using mn 20 step 1 to cause the offending mental fabrication to generate the appropriate amount of disgust itself.