I cook them untrimmed, but here is how to trim them.
"How to cut St Louis Style Spares (Tutorial) - The BBQ BRETHREN FORUMS."
For the membrane, tease up a corner near the middle of the ribs, grab it with a paper towel, and pull it off. Place them on the grill, and cook till a toothpick inserted meets little resistance. I cook untrimmed spares at 250 degrees for 4 hours, then check them.It will take longer each time you peek.