Hello
I learned it with this guide: https://developer.valvesoftware.com/wiki/Creating_a_Curved_Hallway
I know im just kinda throwing a you a google result. Some pointers i found on my own was that the result gets better if you stick to grid, and moved the vertices a bit after so you have nice snapping.
Im also playing with a mod for BM:S but using their included Hammer. I read there were some issues with Hammer++, is it worth it?