Sometimes, you learn the hard way. One tool I discovered in Minecraft Pocket Edition was the bucket. You have three of them. One is an empty bucket, one is a blue bucket, and another is a red bucket. Dumping on the bucket with blue liquid generates water. Dumping the bucket with red liquid generates lava. You can probably see where this is going, right? I made a moat for one structure with lava, and the lava ended up burning a lot of stuff around me. So I quickly did some editing and replaced the lava with water. I kind of like how the water can go from rushing to calm depending on how you place water. The water starts out rushing early on, but when you modify some of the rushing water, it all calms down. One thing to be careful of is that you don't accidentally flood some parts of your world using water.
My world that I am using and creating with doesn't have a true desert. It does have sandy beaches and a few sandy islands, but no true desert. I'll have to find some land in my generated world and make a real desert. When I found some snowy lands, I somewhat found out how to make snowy terrain. However, it is not as precise and real in making snowy terrain. Getting rid of some snow in areas just gets rid of the snow. But when you make grass blocks, you can't make snow blocks as easily as you could with grass blocks.
The current version of Minecraft Pocket as of this post is v0.7.1 alpha. I'd like to see the mobile version have a lot more of the features PC Minecraft has, but I'm sure it will be much different trying to incorporate many things into Minecraft Pocket. I am still fooling around with a number of features. I may next try out some of the plant options and the different eggs.
[UPDATE] Using the hoe on grass in Minecraft Pocket Edition allows you to harvest seeds as long as that harvested land isn't turned into dirt. I just found that out when I posted this update.