First, you need to create an object in Lemur and assign a MIDI message. Using the In App Editor, create a Fader object. Tap and hold the Fader to bring up the inspector panel. Tap on the 'x' variable in the Output section at the bottom of the inspector. Tap Automap in the top-right corner, this will automatically assign the next available CC on MIDI Target 0. Open the settings panel by tapping the top-right icon in Lemur. Tap on More Settings... and look for MIDI Target 0. Select a MIDI output, for example MIDI Daemon 0 if you are using MIDI over Wi-Fi. If you are using a hardware MIDI adaptor such as iRig or iConnect MIDI, select the appropriate output port.
To make simple controller assignments, follow the instructions provided by Apple.
Logic does not support sending controller feedback to generic MIDI controllers. To enable bi-directional feedback, you will need to emulate one of Logic's supported Control Surface protocols, such as Mackie Control. There are a number of Mackie Control templates for Lemur in the User Library.