Transfusing a Pack Red Blood Cell

The ABO grouping or blood typing, they look at, especially in say hematological patients who are transfused a lot. And it's more important in these kind of populations because any sort of a hemolytic reaction will be more troublesome for them. In most people we just do the ABO grouping and then the indirect rooms test all the cross match and then transfuse them. So if we are going to transfuse this patient swap nil. Can you tell me a bit about what's involved in transfusing so what we get them a pack red blood cells?