Enacting policies that violate human rights and treat them as having less value than other people.
So for example, when Republicans use "public safety" as grounds to ban trans people from using the appropriate bathroom, they're essentially saying that trans people can not be treated as having self-control and possessing uncontrollable sexual urges, and they must therefore be kept away from cis people, particularly women. That's a dehumanising position to take, and relies on dehumanising assumptions.
Similarly, when refugees who follow the legal (both under US and international law) process to try to claim asylum within the US, but are barred on account of "risk of terrorism", the logic is that these people came from a dangerous place and are therefore dangerous. That the barbarism they're fleeing is not merely a product of the sociopolitical circumstances of their homeland, but of a failure of character of the people of that land as a whole. The default position is no longer "these are decent people fleeing violence", it's "these people are coming to spread their violence here". That second part is usually paired with complete ignorance of the process of applying for asylum - you can tell who has no idea what the hell they're talking about by seeing who says "we don't vet them at all". Even a quick google search would tell them that it can take 18 months of background checks through dozens of agencies to get from a Syrian refugee camp to being allowed to board a plane, or that the approved way for Central and South American refugees to claim asylum is by showing up at a border crossing or turning themselves into a US authority having crossed the border, that these are parts of the system and that people doing them are already following all of the rules. So again, the assumption that they must be criminals or terrorists, that the violence of their homeland is inherent to their person, is another dehumanising position to take. The way Republicans talk about them makes them seem like rabid animals sometimes, full blown dehumanising rhetoric, to go along with dehumanising policy.
When someone is treated as lesser, when their dignity is stripped from them and are not treated as innocent until proven guilty, that is dehumanising. And it is clear, that the modern US Republican party advocates for exactly such treatment for groups including but not limited to trans folk and refugees.