That's a pretty vague question but I'll take a crack at it. There are three parts.
1. Design the unit statistics and behavior you want on paper/spreadsheet
2. Implement the XML and any python or SDK changes
3. Create the unit art
Anybody can do #1. Many proposed mods get this far and no farther.
If you are a good programmer, you can pick up enough about XML and python to do #2. I have done this; it took about a month of part time work (100 hours?) to get from "no knowledge about civ4 modding" to "having a released mod".
For #3, I have no skills as a 3d artist and you didn't mention if you have any. I know that expert unit artists who contributed to my mod could turn out a new variant unit in a couple of days, or a couple of weeks if the underlying model is completely new like the tanker or motorcycle in my mod. Your milage may be totally different, or zero
If you are not a 3d artist, your best bet is to design something simple (#1) and get enough working to release a playable mod (#2). Then try to get unit artists interested. If you do #1 only and then ask for unit artists, you will probably not get much response.
Theoretically, I suppose if you are a 3d artist, you could do the reverse (#1 and #3, then recruit a programmer for #2).