D-sottering can be a bit of a challenge depending on the board and components you're working with, but for through whole parts like these mechanical key switches, a D-sottering pump makes things pretty easy. First, prime the pump so it's ready to go. Then heat the target joint with a soldering iron briefly. When you see the solder liquefy, quickly remove the iron from the joint. Place the pump tip over the joint, ensuring it has a good seal, and then press the button. Keep in mind that all of this is one swift action, so the pump can suck up the solder while it's still in a fluid state. Like so. Voila! If the solder's not coming off, try adding a bit more solder to the joint. It may be counterintuitive, but this will help the solder retain heat and stay fluid a bit longer, improving the pump's chance at success.