How should a water heater drain pan be installed on a concrete slab floor?
Installing a pan under a water heater is always a good idea.  As a contractor, I have responded to many leaking tanks over the years and a drained pan would have made the situation a lot cheaper and easier for the customer. the leak is one thing, but the collateral damage is another.  If you have clearance above the water heater, you could raise the level with a few concrete patio blocks so it could drain down to the shower drain. If you have a sump hole in your floor, that is a good option. If using a shower drain, this assumes you can tap into it above the trap. Plumbing it into the washer drain might be tricky because you would need a pump to lift the water to the drain inlet.  Not seeing your set up makes it difficult to say for sure. There are also simple alarms that sense water and sound an alarm etc.  This would give you a heads up before the pan over flows and floods the floor.