Simply edit all parts of your lightsaber to get this achievement, that includes:
1. Color
2. Emitter
3. Switch
4. Sleeve
5. Material
Once you have changed all of them from the default option to one you have unlocked then you will get the achievement.
You can access a customisation table in the Mantis or also found on some of the worlds you visit.
I created this video to help:
Hope this helps.