How do you pronounce 'in excelsis'?

How do you say "in excelsis"? You know, Latin for "in the highest," like in "gloria in excelsis Deo" or "hosanna in excelsis," quotations from angels in songs we seem to hear a lot in December. Since it's not English, a common assumption is that the way it looks to English speakers is wrong. So even if you grew up singing it just like "excel sis," as an adult you've probably decided that must be incorrect. But what's the right way? Well, since the c in cælis and other Latin (and Italian) words ...