How can I loot larger amounts of gold?

You must build a raiders hall if you are a raider. You will find this in your build tab under Class buildings

You can only raid gold with your class legion. Your other legions will only take resources.

If you use T1 troops you will have a higher load capacity.

Upgrade Training ground for class legion and 1st and 6th skills of heroes in the class legion as was said above

It is better to explain the steps to activate the raider class. I am still confused and unable to reactivate the plundering with the above explanation.

Gold is only looted by a Raider class when using the Class Legion to attack enemies. The amount you take depends on what is available and on the load capacity of the legion. To increase the capacity have more troops in the legion. (upgrade the training ground and use heroes with upgraded 1st and 6th skills). If you use Cavalry they will have a higher load capacity than other troop types.