How do you make a swamp in Minecraft

Unless you have mods, It is difficult to create biomes (for example, a forest, swamp, desert, etc).
Swamps have a different shaded block for dirt, and it isn't possible to get that shade in other biomes.

The easiest way to get a swamp is to walk around and find one. Use creative mode, if you have it.

If you need to have a swamp nearby another area, use command blocks to teleport you to the swamp and back.