Not much prep needed, if any at all. If the wood is uniformly flat and smooth, with no bumps or undulations, it's likely ready to go. If it's dirty with some embedded grit or grime, then some sanding can clean that up. A relatively coarse grit made for wood-sanding, like garnet paper at 100 - 150 or so, works well and is coarse enough it'll be easy to see (& feel) if any of it's grit is left behind. And the garnet is somewhat less likely to leave deep scratches on your blade (like SiC/AlOx paper can), if for some reason it's left behind on the strop. That doesn't happen much anyway, as the grit is easy to brush or vacuum away from the surface, after sanding.
Planing the wood is very handy when it comes time to replenish them after some heavy use, or if you want to try a different compound on them. Planing will completely remove the top layer to some depth, so there's little risk for contamination by a coarser grit left behind. Also works well to remove any wood filled with whatever emulsion might've been used (oil, etc.), leaving it completely clean and as-new, and impressively smooth & flat as well.
