Method 1) The usual. Insertional mutagenesis. This is much slower and can takes years. The virus uses its strong viral promoters called long terminal repeats (LTR) to switch on adjacent quiescent cellular proto-oncogene, eg by FeLV. Cellular proto-oncogenes are present in all species and have a role in differentiation / growth but are then switched off except during cancer.
Method 2) Very rare. The virus has an host oncogene in its genome and this viral oncogene becomes integrated into host cell DNA and the virus can cause tumours in weeks. Myc, which encodes a DNA-binding phosphoprotein, is one such oncogene. Myc was first discovered in an avian myelocytomatosis leukaemia virus.
Myc is also a mammalian cellular proto-oncogene which is activated in many non-viral human cancers as well as by insertional mutagenesis.
A range of other mammalian oncogenes were first discovered in avian leukosis viruses and include src - sarcoma, myb - myeloblastosis, erb- erythroblastosis.