First find a Poison Lake. They are quite common and can be found almost anywhere in

Poison lake.

The Nether.  Second, craft a Venomite Bucket out of Venomite Ingots (in the same way you craft a bucket out of iron).

Crystal Cauldron

Thirdly, pick up some poison with your Venomite Bucket. (This cannot be done in creative mode).

Then, craft a Crystal Cauldron (same recipe as cauldrons, only with Nether Quartz instead of Iron) and fill it with poison. Finally, right click on the Crystal Cauldron with an Ichorite Shard to purify the poison into water.

A cobblestone generator can also be made using poison instead of water. This is useful for making things such as brewing stands in The Nether.