Dior sizing is fairly simple. 31" MIJ 19cm raws will measure a little less than 17" across the waist. Once you know your raw size, you take it from there. Generally, you'll want to size up 1 from your raw size to the washed sizes. Gets a little tricky w/ MII v. MIJ and 17.5, 19 and 20cm, but not as much as people make it out to be.
When I got my MMMs in the mail and I took out the shoebags, at first I was like "what the fuck is this bullshit" and then I remembered that MMM sa's wear lab coats, and the shoebags are supposed to be those things you put on your shoes so you don't get poop or blood on them or some shit.