you need to kill troops from people who are from another kingdom (you can kill peoples troops from an enemy kingdom if they are in your kingdom for a cvc/kvk etc), also killing  joten troops by defending your stronghold siege is another way to get soul shards.

killing troops from your own kingdoms players however will not get you soul shards.

Sometimes soul shards appear in the bank offers,but personally i would not buy them,but it is another option if you dont care about spending

You can also earn soul shards by activating Stronghold(SH) Siege. SH needs to be lvl4 or higher.

Can be activated anytime and lasts for about 2 hours then can't be restarted for a few days. Best to try out between CvC and all KvK events so that you can practice timing of sending and recovering reinforcements.