Make sure you're logged in, so that your inventory is loaded
Select a unit you want to make a build for
If you plan to use an attack with innate element(s), you can select the corresponding element(s) in the "Skill innate element(s)" section
Fill information on the monster you're facing, its race(s) and its elemental resistance (negative number for elemental weakness). Take into account the elemental debufs you'll give it (for instance, if using Orlandeau's Divine Ruination, substract 50 to the light elemental resistance).
Click on "Build !" button to start the computation
Some features are still missing for now. It wont automatically use Equip X materia (but you can pin one to use it), and Hybrid damage are not yey supported (coming soon).