Souls are a rare and valuable currency in Demon’s Souls. From purchasing new equipment, repairing old equipment, or upgrading your character’s core stats, souls are needed for it all. For me, personally, I might farm small amounts of souls at a time if I’m just under the required amount to upgrade one of my stats, or if I’m just quickly purchasing some grass or spice. It just so happens then that I stumbled upon a very quick farming method whereby I could farm 5000+ souls in under 60 seconds (even more if you equip the ring of avarice). As far as requirements go, a spell (fire toss or soul arrow will do) is virtually all you need. Though, bow and arrows work just as well, albeit a little slower.
Also, to quickly teleport back to the nexus, I recommend using the evacuate miracle. If you don’t have that, you can just run back to the archstone (it’ll add an extra 10 seconds or so). If you’re farming larger amounts, you’ll want to run back to the archstone anyway and select 4-2 again to reset the level. Well, the method takes place at level 4-2, The Ritual Path: