This guide assumes your phone has a S5PC110/Exynos 3110 SoC
Before doing anything, you will need to know the codename of the device. You can find it out on CyanogenMod Wiki or on CyanogenMod download page.
For instance, the Nexus S codename is: crespo.
First thing to consider before starting a port, when all of the above is assumed, is to see how many non-free components are required by CyanogenMod.
The easiest way to do this is to spot the device repository in CyanogenMod repos and look for the extract-files.sh or proprietary-blobs.txt file.
For instance, the list of non-free components for the Nexus S is extract-files.sh
From that list, spot what is related to what hardware component (audio, camera, sensors, gps, modem, etc): that gives an idea of the amount of work required to add support for the phone.
During the port, you might need to find precise infos about the hardware that is in the phone. A good to do this is by looking at the kernel defconfig for the device, another way is to download the Service Manual for the device.