Best course of action is to begin with the belt. As long as that bugger is still fastened, you've got no chance. If making an emergency visit to an article of convenience, we suggest moving quickly before soilage occurs.
There is no need to rip the belt from the loops. This would be a clear example of overkill. It only happens in Hollywood, and sometimes Canadian, productions of the lowest quality. Avoid it.
Once the belt has been adequately loosened, your work is not quite finished, unfortunately. If you have the misfortune of a "button fly" you will need a lot of manual dexterity and a heaping chunk of patience to get through the task ahead of you. Hopefully you had the foresight to procure trousers with a normal, human zipper. Even so, you are not free of the evil clutches of the Button. Deal with it in any way possible. If thumb and forefinger fail you, feel free to resort to scissors, steak knives, chainsaws, or one of those creme brulee flamethrower thingies. Whatever it takes to get the blasted button undone.
You're in the home stretch. But don't rest easy just yet. That could result in social ostracism, and an inexplicable mess around your ankles. The zipper! The zipper, man (or woman, or however you self-identify -- doesn't matter much at a time like this)! Take the zipper down at a pace that feels natural and comfortable to you. Once down, you have the "all clear" to slide the trouser object as far down as necessary for you to perform whatever task led you to seek out trouser removal instructions in the first place.
Bon chance, amigos.