Only healthy dogs must be taken for mating. This means that both the male and female must be free of mange and ectoparasites (fleas, ticks, lice, etc). Similarly, neither animal must show any overt symptoms (vomiting, diarrhoea, inappetence, etc) of any ailment immediately before taking them to mate. Their vaccination status must be up to date. Finally, if you have to carry out any helpful intervention, please be careful. For example, when clipping hair from around the male dog’s penis (see below), one might cut the foreskin. This would cause pain and reduce the dog’s desire to mate.
I have found the following text, largely put together by Drs Carlson and Giffin, to contain sound advice on the preparation of the two dogs prior to mating.