Haven't played this game for a few years and don't have a clue as to filling a soul gem. I need to fill two of them with ash ghoul souls. I am open to a little help. thanks

Once you find yourself some soulgems big enough to hold the soul, cast a short duration Soul Trap spell on an ash ghoul and kill it before the spell runs out. If you succeed, you'll get a message saying you captured a soul. That's it!