If you are trying to back up your cartridge save for Gen 3 there are a few things you will need. The main devices I will be using in this tutorial are a DS lite (you can also use an original DS) and some sort of a DS Flash Cart (R4, DS Two Plus, AceKard2i). Note: if you do not have a flash cart, you can also use the exact same tutorial method for Gen 1-2 cartridges. This will require a Retron 5.
If you're looking to manage saves for other Gen 3 games on the Gamecube, refer to this tutorial