Honestly, you can use any method you are comfortable with.
I have some of mine I maintain on the strop (and sandpaper if necessary), and some I just use my diamond stone and ceramic sticks on.
^^ THIS ^^
I think it is useful to distinguish between a convex
GRIND (describes the shape of the blade from the spine to just above the cutting edge) and a convex
EDGE (describes the shape of the blade, well, at the cutting edge.
The Opinel has a convex
grind. You may or may not decide to put a convex
edge on it. Nothing wrong with putting a conventional 'V' shaped edge on it either.
If I have the time, I use a Lansky system and put a 17-dps back bevel followed by 20-dps primary bevel (aka micro bevel). This is a crude but easy and replicable approximation of a convex
edge. Very often, I'll touch up the knife on a diamond pocket stone which will somewhat guarantee a bit of convexity to the edge just by my hand motion.
The only bit advice I can give on sharpening an Opinel is that I find the edge holds up better at around 20-dps. I wouldn't push the carbone any thinner than that and the Inox only if you are willing to maintain it.
Beyond this, don't worry about it. Just sharpen it the way you normally sharpen. The convex
grind is very subtle and honestly, I don't think it affects what's going on at the cutting edge much at all. I does affect how the blade goes through material like potatoes but that is happening well north of the cutting edge.