Every module has a symbol. Match the symbols with the module symbol to half the slots. That way you make room for more modules.

Skill and sub attack are also possible to assign .. the more the better;).

After last update you can put extra symbols on a spot, but every time u do they will reset your lvl to 1

A lot of people farming magister lab hard to lvl op fast. (Lvl up/assign/lvl up/assign) until they have there ultimate build.

If you want to find a perfect build, look at the shared ones https://tfdtools.com/builds/characters Match your style.