Hi there! I can certainly understand wanting to have little Yorkie puppies!! But, before you breed your dog with your friends dog I want to encourage you to have all the pre-breeding tests run. This is very important for the health of the pups and all the future pups that are likely to be born from this mating.
It's sort of a rule of thumb around the canine world that you should only breed dogs that are a good representation of the breed. Are both Yorkies good representations? Not just in looks, which is VERY important, but also in build and health, which is even more important. You want to make sure you are not passing on any genetic problems to future generations such as luxating patella (my poor Sophia has that and will have to have surgery.)
I am not a breeder, so I can't offer you any help with them mating, but just wanted to offer you some advice before you actually mate them.
Best of luck to you and your dogs.
(maybe you've already done all the testing and they are great representations of the breed...and if so, at least someone else reading this thread might benefit from the information.)