Whoh..... First of all I am glad that you are asking questions.
You have a whole lot to learn about the yorkie and breeding before doing it. There are some great books out there about breeding and some great ones about yorkies (you usually cannot find the best ones in pet stores)... One good one is The Complete Yorkshire Terrier by Gordon and Bennett. There is a little section in there about breeding too.
First off you have to make sure that your yorkies are to standard and worth breeding, as well as test them to make sure hopefully they are sound and healthy enough to be breed and not pass on bad problems to pups. A regular health exam is not enough to find out. There is more to it, such as blood tests, petellas exams, etc...
You also need to check their relatives history - what is in their generic make up. How are their parents, uncles, aunts, grandparents, etc... You want to know as much as possible about their family tree, so you can make sure nothing bad might pop up in their pups. Breeding is very serious business and you need to know as much as possible for the pups' sake and your dog's sake.
You cannot just put them in a room together and hope they breed - this is dangerous and not good for either of them. So you need to get some books and do your research and learn. Just being on YT is not to best way to learn - reason is because there is so much to it that hitting a key board can do in a few minutes.
Best of luck to you and happy research. Research and learning is a lot of fun and very interesting. I am sure you will enjoy learning.
Here are a few links about the Yorkshire Terrier Standard:
http://www.akc.org/breeds/yorkshire_terrier/ http://www.ytca.org/mainview.htm http://www.ytca.org/standard2.html