Depends on what you are trying to do. I gave a mouse line cancer by introducing a (previously unknown human oncogene) into the mouse, then waited to see if they developed cancer. A mouse line is a line of mice that are genetically identical to each other, not like mice you catch in the field which have genetic variation between them, particularly with regards to the immune system and what it recognizes as "foreign". This was a transgenic mouse, meaning we could introduce the gene into the fertilized egg, then implant it into a mom mouse, the mouse grows up, and we watch and wait. The nice thing about this is we could put a human gene in the mouse in this way and not cause immunological rejection. Had we taken human cancer cells from which this gene was derived, injected them into the same mouse type, the mouse immune system would kill those human cells. In my case, 100% of the mice that had the gene got cancer, and quite rapidly at that. That is one way. This is not a fast thing to do, may take a years to set this all up, then when you get some with the gene you need to breed them a bunch to have enough, and also keep an eye on them to check for signs of cancer. For example my experiment took 2 years from start to finish and it was that fast because the cancer gene caused cancer fast.

OK you maybe want to use mice but you need to inject human tumor cells. Like I said in normal mice they have immune systems and they will reject human cells as foreign, with the immune cells killing the human tumor cells (or human non tumor cells). You might say, but they are human tumor cells, how can that be? The cross species immune reaction is pretty strong and it will quickly kill any cells from a different species very quickly, even cancer cells. In fact injection human tumor cells into another human it is highly likely that other persons immune system would kill them too. If that other person was an identical twin? Different situation, those cancer cells may well grow (I say "may" since ethically can not do this experiment, but can do it in animals), might be a darn good chance they will grow in an identical twin. Back to the mice since we don't experiment on people, what can we do? Well we also have mice that were bred to not have immune systems. So when we inject the human cancer cells in them there is no immune cells to kill the human ones. A lot of the times (depending on what you are doing) those human tumor cells will grow, not always (reasons complex, too long to explain), but works pretty good. Now you have a mouse with human tumor cells growing in a mouse and you can do experiments.

There are other ways this can be done but these two are pretty straight forward and easier to do.

This is with a lot of simplification to keep things simple but describes some of the simpler things we can do.