Before you start, you must ensure the control you are trying to map as an encoder really behaves as an encoder. The difference between faders and encoders:
Faders: outputs discrete absolute values in a range (between a maximum and a minimum)
Encoders: outputs relative values (increment or decrease)
In the MIDI world, sometimes there are devices that have physical encoders, but will output data as faders.
So, being an encoder means, in general, that when you rotate the physical encoder to the right, it keeps outputting the same value (with no limits). Rotating the encoder to the left, will output a different value. The increment/decrease value depends on the number of encoder "ticks".
Some MIDI devices allow you to edit the controls output. Some of them can be set to work as encoders or faders using a manufacturer's application. Please check out your device User Manual for this.
Another workaround is to set the minimum "fader" value of 0 and maximum of 1. This will turn your control into an encoder-type because rotating to the right will make it output 1 and rotating it to the left will output 0.
If you cannot edit the control output to make it behave as an encoder, you can still map it as a fader and then use the Fader to Encoder driver.