If you're uncertain of how to in-plant a cutted part of a stem-plant...........you can ALLWAYS take the length you want (beginning from the growing end) and cut. Then remove one or two lowest leaves. You now have a cutting with one or two nodes (growing points) left, where you removed tbe leave(s). The cutting is planted so these nodes are under the gravel/soil, and the rest is free in the water. The nodes are allways more willing to produce new roots, then the internode (the piece inbetween nodes = stem).
A lot of stem-plants happily produce roots directly from the stem, but not all........ to be sure give the cutting one or two nodes to produce roots from.
- and yes, it's the very same nodes, that can produce new shoots too, when you cut tbe top off the stem 😉
Mick.