Assuming this kind of therapy is effective in humans, the rate-limiting step is identifying a protein in the cancer that is either not produced in normal cells or sufficiently mutated relative to the same protein in normal cells, such that the immune system targeting that protein will only kill cancer cells. Otherwise you'd just induce autoimmune disease.
Are you asking how to identify such a target protein? Well, you could sequence cancer peptides using mass spectrometry, or sequence the cancer cells' genome, and look for proteins that are sufficiently mutated relative to the wild type copy. However, we already have pretty good knowledge of which genes tend to be mutated most often in cancer cells, so you could do targeted sequencing on those genes first to look for low-hanging fruit. In general, it's still a hard problem, though.
