All the other questions I've seen so far are a step ahead of me. I've defined an Input for a vector layer, and am trying to use the Select by Location Algorithm. I typed the name of my Input, but the modeler doesn't seem interested in connecting it. Any ideas?
Update for QGIS 3.14 - from QGIS graphical modeller joining input layers to processing algorithms
To select something else that a layer in the current project - such as model input, or output of another algorithm in the current model - the category of input has to be defined first using the '123' button: