How do I prevent thread from tangling on my bobbin?

Keep your bobbin case clean and use a thread guide - that's the biggest thing most people skip. Lint and thread bits build up in there over time and create snags that make new thread tangle. Empty it out, wipe the insides with a soft brush, and make sure your bobbin sits flat and centered. Also check that your bobbin itself isn't damaged or warped; a chipped edge will catch thread every time.

One hack that actually works: wrap a thin piece of tissue paper around your wound bobbin before you load it. Sounds weird, but it creates a smooth surface so the thread doesn't dig into itself as it unwinds. Just tear it away when you're done. The other thing is don't overwind your bobbin - leave a tiny bit of space at the edge. People cram it full and then wonder why the thread binds up. Fill it until it's maybe 80% full, keep your winding speed moderate like the previous answer said, and you'll have way fewer headaches.

One thing that actually helps a ton: wrap a tiny piece of masking tape around the bobbin's edge where the thread sits before winding. It creates a slight lip that stops the thread from sliding around and bunching up on itself, especially if you're dealing with slippery synthetics or metallic threads.

When I was dealing with a tangled mess on my machine last year, I realized the thread was bunching up because I wasn't securing the tail properly before winding - you gotta hold that starting end down firmly so it doesn't slip and create loops underneath. That single fix stopped most of my problems right there.