How should I prepare Bok Choy before I cook it?
I've never tried steaming bok choy, but yes, the ribs and leafy bits cook at different speeds.

For stir frys I trim the bottom so the ribs come free, clean off any dirt that's gotten trapped in between, then stack up the leaves and cut the leaves from the ribs.  I slice the ribs into about 5mm / 3/16" slices, and add them towards the end; the leaves I cut into ribbons (perpendicular to the ribs), and stir them in at the last second, so they barely cook.  The center I just chop the top off the leafy bit, and slice the more rib-like bits a litte larger than the rest of the ribs (they're thinner, so cook faster).

As I've never tried steaming it, and I don't tend to cook in bags (except for silver turtles when camping), this is going to be a complete guess.  Now, it tastes fine al dente, so you could leave the ribs as large as I use for a stir fry, but en papillote tends to be fancier cooking, so I'd probably try for maybe 1/6" slices or a little thicker (maybe 2-3mm), and if you had a mandoline so they all came out even, that'd be great.  I'd still cut the leaves into ribbons, just because it's easier to eat than a huge intact leaf.

I'd assume that the leaves would cook at a similar rate as spinich, and the ribs closer to fennel, if that helps at all.