Yes, I think you're right - we'll make sure to add a screencast for the next release. 
The procedure for 1D interpolation is the following: 
- Load a point map with the data of the known points (e.g., gauging stations) to the maps panel.
- Load a line map with the 1D geometry (e.g., river). 
- Use Link to parameter on the point map and link to the corresponding boundary condition. 
- In the link properties, choose the line map as the selection map. 
- Also in the link properties, choose 1D Linear Interpolation as the interpolation method.