To modify your lightsaber you need to find a workbench to do this. Lucky for you there is one located in the back of your ship, Mantis, which you will be cruising around on after the first tutorial mission of the game. If you are on the first planet Bogano there is also a den you will discover that has a workstation to customize your light saber.
1) Lightsaber parts are found in chests scattered throughout the various planets. Alternatively, if you have the pre-order bonus content you will have all the parts to fully customize your light saber already in your inventory.
2) Open the work bench and select each section of the saber and swap them out with a new skin, metal, saber color etc.
After this the achievement should be yours. Good luck!