If you already have existing MIDI tracks, you can either 1) use them to trigger one of your external keyboards, or 2) copy them onto a software instrument track (and have them play back a "virtual instrument").
1) For external, you connect your MIDI cables between your interface and your keyboards, and then connect the audio outs from your keyboards onto an audio track in Logic. When you record enable the audio tracks, the MIDI track will "play" your external synths, and the audio out from the keyboards will get recorded into Logic.
2) Alternatively, you can create a software instrument track, load the sample sound (the virtual instrument) you want to hear, and then copy the existing MIDI track onto your new software instrument track so it will play.
When you are finished with these first steps, you bounce ("bnce" from the 1-2 main out in the mixer window) the audio to a wave file. Then you close Logic, start Waveburner, and load the resulting file into Waveburner. Last step is to burn your CD. "Burn" CD is a selection from one of the Waveburner pull-down menus. It's a lot easier the second time than the first!
If you decide to work with external instruments, they may need to be "defined" in the environment. They may also need to be setup within themselves to transmit or receive on specific MIDI channels, especially if you want to deal with multi-timbral sound modules. This is also something that is more difficult the first time, than the second. A quick intro: 1) open the environment window; 2) select the "MIDI inst" layer; 3) choose "new" from the pull-down menu and select "new Multi-inst"; 4) remove all 16 slashes from the icon, to activate all 16 channels; 5) name the Multi-inst.
You can then select it from the arrange window, to direct the MIDI signal from Logic to the external synth.