Ok, so now you have some good Gold Historic generals and you want to know how to most effectively put skills on him. First thing you want to do is decide what three skill you want to put on them. After that you will want to acquire several of the level 1 book for those skills.
Then you begin the painstaking process of getting all three on the general. Its best to have several of each as they have a very high chance of replacing each other instead of filling an empty skill slot. You can buy skill books in the skill book shop in your academy.
So after you get all three to stick you are past the difficult part and onto easy times. Now you will simply put on the level 4 version of the three skills. A level 4 book will 100% without a doubt replace the level 1 version of the same skill without risking replacing one of your other skills. This means you just simply add the level 4 books and you are done! Below is a Youtube video I found by 'The Miser' that shows you exactly how that works if you are a more visual learner! Keep in mind that his video was made when level 3 was the highest level skill book so he does not mentions level 4 skills at all but the same principal still applies.