I grabbed a pupusa recipe out of a stray NYTimes Magazine and made it for the second time last night. It's pretty easy, take two parts masa harina, mix with a bit of salt and one part (hot!) water. Mash it together, divide it into balls somewhat larger than a golf ball. Then somehow get cheese (about 6oz per cup of masa) inside of them and press them flat.
The last bit is perhaps the most tricky bit, and I haven't quite mastered it. The NYTimes has a technique that you can study more closely here. Mostly just mash the ball flat in your palm, insert the cheese, and then squeeze the masa around the cheese and seal in the cheese. Then squish the final result flat and cook it up on a griddle.
It's a nice Sunday recipe here because halloumi goes very nicely with these and can be found at the local place that is open on Sunday.
The NYTimes serves a curtido with them, but we're more likely to have sauerkraut around, so we use that instead.