The first step is to select the longest path of the river for which you want to calculate the time of concentration. Unfortunately, there's not a simple tool that does that. Here we'll use a manual procedure.

1. Start QGIS

2. Open the Kirpich project from the GeoPackage provided with this tutorial. In the main menu choose Project | Open from | GeoPackage...

3. In the Load project from GeoPackage dialogue browse to Kirpich_data.gpkg and choose Kirpich as project.

The project has the catchment polygon (catchpoly), channels, DEM (SRTM 1-Arc Second) and is styled using blending with a hillshade.

4. Click on channels

5. Click the Select Features by area or single click icon

6. Now select the channel segments of the longest path from upstream to downstream. Keep the <Shift> button pressed to add the segments. Zoom in if necessary. You can use the scroll button of your mouse to zoom in/out and you can use the spacebar in combination with moving the mouse to pan. If you accidently have selected a wrong segment, click on it again with to deselect it.