There are many ways to induce cancers in the rat, and it depends on what is being studied.
Sometimes you can just alter their cells with some enzymes causing them to split differently. Other times you can implant a chip or some sort electrode, or even a radioactive chemical to an area to cause the cancer. Again, it depends on what they are studying and why.
There's no one single path to cancer in lab rats, but injection is another common approach that may be easier to wrap your head around. Somewhere like
American Type Culture Collection may be a good place to look. They are a national repository for cancer cell lines, and you can probably get some of your questions answered there.
If you wish to do research like this on your own, note that you will need also a Biosafety Level 2 Lab to prevent contamination in your study's cell lines.
EDIT: I forgot inhalation, like for testing smoke and asbestos.
