You need to shave a part of the fur on the back and make a injury on the surface by scratching. Then inoculate with your bacteria and after covering inject your antimicrobial intravenously or by topical administration. Another option is by intradermal injection of the bacteria (google: experiemental skin infections). Some researcher create a flap in which they inoculate bacteria.
We first shave the mice on the back and then apply depilatory cream. The bacteria were injected intradermally in a volume of 50-100µl. The size of the lesions depend on the amount of bacteria. You have to test this with different amounts. We use about 1 to the power of 8 bacteria.