Will second this. When modding Xbox 360s, I used MrMario's guides and he would say repeatedly "clean, flux, tin", kinda stuck in my head. I did also tend to just clean the whole board while it was apart, but especially the point you're about to solder should be clean.

I have never used sandpaper on electronics, but I perhaps similarly use a fiberglass pen. Total game changer for getting old cartridge pins to read again for SNES and GBA games and such. Highly recommend picking one up.