I carefully scribe a fine line on the key so the slightest movement can be instantly detected. I deburr the keyway without enlarging the keyway appreciably. The key end of the drift must have at least the same area as the head of the key including the tang. This is important as the idea is to transfer the blow of the hammer on the drift over the entire area of the key thereby minimizing the potential for serious damage to the head of the key. The next step is to slide the drift into the keyway up against the head of the key.
Hose clamps make good straps around the shaft to hold the drift in the keyway.
Start out with a easy rap on the drift. If it has then use one of the penetrating fluids around the key. When it has moved at all this means the rust bond around the key is broken and a penetrating fluid will then easily flow into the joint. I have had little success with getting penetrating fluid into a rusted joint unless it is first cracked. Do not drive the key in further.