how to make water drain against gravity?

You cannot "Drain" against gravity. the closest you can get to that is a syphon. A syphon will lift up to ~32 feet. Higher than that requires a pump.

By raising the water tank up off the ground.  Then put in a hose and lead the hose from the water in the tank down to ground level.  Then suck on the hose until the water starts to come out through the hose.

That water will continue to run (without you sucking) until the water in the tank lowers to below the end of the hose that was in the water.

So the water in the tank is lifted up against gravity, over the lip of the tank, and then it goes down to ground level where it flows out of the hose.

What lifts it up over the lip...air pressure pushing onto the surface of the water in the tank.  When you suck on the hose initially, that lowers the air pressure in the hose to that there is a net force between the surface of the water and the hose.  And that's what pushes it up against gravity over the lip; once it's over the lip gravity does the rest and pulls it down the rest of the way through the hose to ground level.

BTW:  This works with gasoline tanks and siphoning petrol out of them. 

To go against gravity work must be done and that normally means using some kind of  pump.

Syphon is the simplest way, then a pump either manual or powered.

Apply a force to it that exceeds the force of gravity. Water will never naturally drain against gravity but it can be pushed against gravity.

Use a pump, water doesn't go against gravity.