Modeling in Houdini can be more about nodes rather than the traditional 3D tools that you might be used to. The concept of procedurally modeling objects will certainly be a foreign one, if you are starting out, or have never seen the process before.
If you kind of know your way around Side FX Houdini, and want to know more about modeling, Alejandro Pérez Sánchez has pretty good tutorial that the process from front to back. Alejandro demonstrates building a fern form scratch. It is really an effort in procedurally modeling plants in Houdini, as you can use these techniques to build any type of 3D plant.
If you wanted something a little more in-depth when it comes to procedurally modeling assets, Tighe Rzankowski has a nice tutorial that you can go through. Tighe covers procedural modeling concepts from start to finish, showing how to create a rudimentary fence asset that can easily be expanded upon for other purposes.