I use Comet or Bon Ami cleanser and a little light oil to make a slurry. Dab a very small amount on the top of the sleeve flange (being sure not to get any down inside the sleeve ) and just push gently but firmly and turn, then rotate 90 degrees and push and turn several times more. Keep this up until it feels smooth...that's the cleanser breaking down and getting finer, and that's your cue to clean and check your progress.

Next step is to clean everything off and inspect...looking for a nice light gray frosted appearance all the way around the flange and receiver groove in the head.
Repeat as (or if ) necessary.

The whole process seldom takes more than a few minutes.

It will work well on a slightly warped head but sometimes you need to do the top of the sleeve flange too. For that you need a flat surface like a pane of glass and some 600-1000 wet/dry paper and oil, then do the lapping.

Edit: After reading the thread in the Glow Engines forum, lapping ain't gonna do a dang bit 'o good here aye!?
Maybe send Mr. Enya an e-mail and a link to the thread(s )? Perhaps he'd send you a new head?