The weapons don't need to be any particular grade, so they can be level 30 of basic weapons. This is actually easier to do if you have the lower tier weapons since they often have 2 or 3 slots. It is much quicker to fill the slots with cores when you have less slots to fill. This will come as you are working toward filling the cores on the level 30 legendary weapons.
Equip weapons and equipment of at least level 30 in every slot, and fully socket them with maximum level cores.