Severe icing is generally limited to an area from a couple hundred to a thousand feet thick....
As far as the question asked, the icing equipment should already be on.....the only other pertinent item is climbing or descending and the only caveat there is what is the terrain like where you are, are you starting or finishing the flight, and after that I would say that you would follow the flight manual/QRH for severe icing encounter if there is one and land using extra speed/lower flap settings due to the possibility of tail stall. (Most transport category airliners do not have tail de-icing.....and the ones that do (the DC-9 comes to mind) in order to get tail heat you have to remove it from the wings....
A350
As far as the question asked, the icing equipment should already be on.....the only other pertinent item is climbing or descending and the only caveat there is what is the terrain like where you are, are you starting or finishing the flight, and after that I would say that you would follow the flight manual/QRH for severe icing encounter if there is one and land using extra speed/lower flap settings due to the possibility of tail stall. (Most transport category airliners do not have tail de-icing.....and the ones that do (the DC-9 comes to mind) in order to get tail heat you have to remove it from the wings....
A350