[Author's Note: While my series of posts on world-building is complete, I forgot to include this one, as promised. To correct this, here's the missing information for all you world-building fans. Enjoy.]
Deserts are unique places you'll want to include in your world-building plans. They occur where there is little to no rain to support grass and moss. Where there is rain, dry grasslands or scrub land result. So it makes sense to place desert next to scrub land or where the land is so broken (like the American Southwest) that only the hardiest plants and animals can thrive.
But all deserts are not the same. They are as varied as forests and grasslands, with their own ecosystems. We'll talk about placing animals and other resources soon enough, but when placing a desert in your world, think about the type of place it is and what lives there.
Consider our own world. The Sahara, with endless dunes is a very different desert from the American southwest which is rockier and full of cacti. In the Sahara, you have oases with palm trees, camels, and humans. Elsewhere there are hyenas and jackals scavenging for food. Snakes and scorpions are also common. If you want this type of desert in your world, think about what else might inhabit the sands.
The American Southwest, on the other hand, is home to some amazing geological formations. It is dry and rocky. Images of the Grand Canyon or the Mojave Desert come to mind as example for these types of deserts. Here, there are snakes and lizards, but there are also tortoises, spiders, cougars, ravens, owls, hawks, and ground squirrels. Since the desert in this area are not on the equator (like the Sahara) they have more of a seasonal fluctuation in temperature. They don't have seasons as most days are dry and warm (or hot) and nighttime can get very cold.
Tundra is another ecologically unique place and if your stories take place in the far north or on mountain plateaus, you should consider add tundra to your world. If you decide to include this type of terrain, place it in far northern climates. On our world that is north of the Arctic circle and on high mountains plateaus beyond the timber line.
Like desert, tundra has very specially plants and animals that live there. Let's look at our own world for examples. In the far north of Alaska and Canada, where it is too cold for trees to grow, tundra begins. Here the land is covered in grass, moss, and lichens. The land is flat here making it a good place for herd animals, such as caribou, reindeer, and musk oxen, to graze. Arctic hares, arctic foxes, migratory birds and polar bears also roam the tundra. Farther north are the ice caps of our world and as the weather turns colder, the snow and ice of the Arctic invades the areas of tundra for the fall and winter burying the grass until the spring thaw.
Tundra also occurs in high mountain plateaus, such as in the Alps, the Andes mountains, and the Himalayas. If you select a more mountainous site for tundra, then the plants and animals living that may differ from the ones I listed. For example, there won't be polar bears, but there are likely to be mountain goats and mountain lions. Adjust the ecosystem of your world as well, placing the most appropriate plants and animals in this cold and harsh locale.
Also feel free to research both types of environments for more detailed looks at them. I've given only a quick sketch here of each type of terrain. You'll want to be more though and when you combine these terrains with other types of vegetation and terrain, deserts and tundra give your world a realistic feel.
While they can be hard to place, I encourage you to do so.