I feed mine smallish pieces of squid, table shrimp (your standard raw shrimp like you would cook and eat), and scallops. You can usually find a bag of that mix for cheap money in the frozen seafood section of the grocery store.
Thaw some of it, cut it into small pieces (smaller than dime size) then pack it flat in a freezer bag and refreeze it.
When you want to feed, break off a chunk, thaw it out in salt water so it is up to tank temperature, then gently set it in its tentacles (I use planting tongs).
When you first start, err on the side of too small rather than too big - I thought mine was going to explode when I first fed him some scallop.
I feed mine every few days to once a week or so (I like to keep him guessing). And he's growing even HUGER than before.
Previously I target fed him mysis, cyclops, and omega one. He also ate cyano one time which was pretty neat.