Grinding seems like a pretty simple thing, especially when it comes to prepping a joint for welding or grinding a weld smooth. With that said, I’ve seen jillions of people do it wrong and the reason is usually because they are using the wrong tools or wrong abrasives. Hard grinding discs, flap discs, Scotch Brite discs, sanding discs, and a myriad of other options exist that will help you remove material when you need to. But there are also non-power options like files that you should have in your arsenal as well.
With all that said, the tools and abrasives themselves are pretty easy to acquire once you know what you need, but then knowing how to use them takes some practice. Don’t use an abrasive on a project that matters if you haven’t ever used it before. That’s what scrap materials are for. Take a bunch of scraps and weld them together and then finish them to get some practice on both the finishing of the welds, but the preparation and the welds themselves. The way to get good at welding and finishing welds is to do it a bunch. Practice really does make perfect, so if you try this out you’ll get better and better each time.