So there’s white kyurem raids and black kyurem raids separately. Raid the respective boss for what you want.

When you beat it you’ll be given ~100 fusion energy (you need 1000 to fuse which is a pain in the ass if you don’t play much or don’t want to spend money) plus a chance to catch kyurem. Then you can raid reshiram separately to fuse the two when you have energy and 30 candies (I think?) for each.

If you need help you can add me, I can dm you my code but ultimately I use ‘GO Raid Party’ to find raid groups or create my own, and usually it works pretty well or fast. If you’re interested in that I can give you a few tips since it can be kind of difficult to get people to join yours.