TA Design Guide [author unknown]
This comprehensive guide from the heyday of TA takes you through all the stages needed to build a fully-functioning unit. Although the sheer quantity of information can seem a little intimidating - and it is aimed towards TA rather than TA:K unitmakers, it's an invaluable reference point if you want to learn how to create a unit or race, and most of the info is appropriate for TA:K too.
Just as a disclaimer I didn't make this at all, just thought it would be a useful link (if anyone knows who did, just shout out and I'll add it to the post!). Forum won't let me hyperlink.