stuck on using lightning gem ds3

If the lightning infusion feels weak on your build, Gold Pine Resin does a similar job without any faith. Rub it on and you get the lightning damage for a while, and you can keep the plain version of the weapon with its normal scaling. It's handy against things that are weak to lightning without committing a whole infusion to it.

You don't use the gem yourself, it goes to Andre. Give it to him once, then go to his anvil, pick infuse weapon, choose the weapon, and lightning should show up in the list. Two things catch people out. Unique and boss weapons can't be infused at all, so if your weapon isn't listed, that's why. And lightning infusion swaps most of the strength/dex scaling for faith, so on a build with no faith it can end up hitting weaker than the plain version. Compare the attack rating on the infuse screen before you confirm.