How can I install a drain pan under my existing water heater?
The only slightly easy way is to drain the water heater of all water. Hopefully the tank is connected to the water supply pipes with flexible hoses and not soldered copper. The tank is now ready to be raised so the new "Smitty" pan can be slid underneath.   Depending on the size of the tank you may want to enlist some helpers. I've managed to wrestle a 30 gallon tank onto a catch pan, but a single family dwelling is more likely to use a 40 or 50 gallon tank. Even empty it will be awkward for 2 people to lift while a third person slides the pan underneath.   I've found it helpful to have some 2 x 4 or 4 x 4 blocking on hand so if needed can support the tank and slowly removed as the pan is inserted.