A couple ways to plant them:
strip off some lower leaves, carefully grab the stem with a pointy pair of tweezers (don't crush it or it will rot) and insert into the substrate. Might not work for very delicate stems though.
You ca also weigh them down with those aquarium lead strips (which do NOT contain any lead btw) onto the substrate until they grow roots down that anchor them.
I wouldn't pin them down with anything heavy: it will just crush the stems, and then they'll rot and float up again. What will keep them in the substrate is healthy root growth, and that takes a bit of time to develop.
Also, when planting those with a rhizome (a sturdy horizontal root from which the finer roots branch), make sure not to bury that rhizome or the plant will die (java fern, Anubias)