Naturally terraforming should be first on the list, because you can use it to create your other maps. If you are not familiar with the process of terraforming it would be best to find a tutorial on Youtube. You need a program/mod called Single Player Commands or MC Edit. I highly recommend the use of a grassland seed instead of a flatmap, so you will not easily hit bedrock. Note: many of the commands may differ depending on your version and/or mod. You can also do [t//undo] at any time. You will want to also have Too Many Items, and switch to creative mode.
Terrain: It can take forever to try to terraform everything in the map, including seas, huge mountains, etc... Most experts use tools like WorldPainter and VoxelSniper to make the basic terrain. My only recommendation is that you don't use them for fine details.
Mistakes: Mistakes are natural and everybody makes them. You can normally hit [t//undo x] at any time. X represents the number of undos you want. You can only undo stuff you did using world edit, not placing/destroying blocks by hand. If you make a lot of mistakes you may not be able to undo all of them. Also when trying to undo the placement of lava/water you may experience a glitch resulting in only half of the substance to be removed. If you cannot undo what you just did the next best thing is to hide it. You can just terraform over it or try to delete it with a [t//Br sphere air 3]. It is a good idea to save and backup your maps in many places.
Arches: Tutorial: To start off you should make some simple arches, everybody loves them and they are easy to make. Assign a stone sphere brush to a tool, with a size of 3 (Note: 3 is a generally good number, yours may very depending on what you are making). Place the first stone sphere so it is about halfway in the ground. Then place then next spheres about halfway inside each other so it looks like it's flowing together. You can hit undo at anytime, so you don't have to be too carful! Make the arch like a crescent, don't flatten it out or make it too tall. Now that you have your stone arch you will want to add some dirt. The next part is a bit tricky for noobs, so be carful. You are going to want to go to one end of the arch at ground level. Hit [t//pos1]. It should set your first position. Imagine you are trying to create a box around the arch. If the first position you set is one corner of the box, you will need to go to the other corner. Keep in mind that if your first position is on the ground your second position will need to be in the air above the top of the arch. When you are there hit [t//pos2]. Next hit [t//overlay dirt]. This will overlay your entire arch with dirt. Depending on how well you set the positions you may have dirt on other things. It will take a while to get the hang of it, but you should practice this step many times. Next hit [t//overlay grass]. This should change all of your dirt blocks to dirt with grass. This step is optional, but it is recommended that you go back and replace some of the stone blocks near the dirt, with dirt to make it more realistic. Now you are done with your arch. Placement: Arches are good in many maps and add a very nice touch. The problem is many to not know where and how to apply them correctly. Never copy and paste an arch from one map into another no matter how nice it may look. Each arch should be tailored to fit the specific place it's going to go. Make sure your map is clear of any trees near the base of where your arch is going to be. If you want to clear vegetation quick I highly recommend mightypork's Magic Wand Tool Mod. When selecting an area for your arch, never pick a flat place. It will not looks good and seem out of place. A good place might include: between two mountains/hills, over a river (make sure the banks are not flat!), from the ground to a cliff/mountain. Make sure your arch fits the scenery, for more info see the section below. Overall people love arch, yet they are relatively easy to make. They are sure to make you maps much more interesting!
Rivers and Lakes: Tutorial: Bodies of water make nice scenery no matter what you are making. To make a pond or a lake you are going to want to make a hole in the ground first. Use the command [t//Br sphere air 2] and bind it to a tool. You do not have to use [2], but I find it is a good number. Now use the tool to dig out the lakebed. Make sure it is deepest in the middle and shallowest on the sides. To make it more realistic at some clay to the bottom. Also make sure there is dirt near the shallow part and stone in the deep. You can put the clay anywhere. When you are done stand on the level that you want the water at and hit [t//fill water x y]. X will be the diameter of the substance you want filled and y is the depth. You must change these depending on the size of your lake. To make a river you will want to hollow a good sized area out, not too deep not to shallow, where you want your river. Next you will need to raise the banks on the river a bit. Then just fill it with water just like the lakes. Placement: Don't make your rivers to big, as people will thing they are weird looking lakes. Also unless you really know what you are doing keep at your water still. Don't try to make rapids. Lakes are good anywhere. Just as a general tip, if your are building on an island, don't make the lake larger than half the size of the island.
Mountains and Volcanos: Tutorial: Making mountains depends on the size and shape you want it. If you want to make a small one just use [t//Br sphere stone 3] to make the general shape. Then use [t//br smooth]. This will bind a smoothing tool to your tool. Use this to smooth out your mountain, until you no longer see the individual stone spheres. Then set the positions and cover it in dirt and then grass. To make a larger mountain, make a ring of stone spheres. Next place a ring of stone spheres that is slightly smaller on top of the existing ring. Keep doing that and as you get closer to the top start making the size of the spheres slightly smaller. It will end up being hollow. You can leave it like that or fill it up (I would recommend filling it up). Go to the center of it on the bottom. Make a column of stone about 10 blocks high. Use command [t//fill stone x y]. X will be roughy the radius of your mountain (it doesn't have to be exact) and y represents the depth. The depth should be whatever hight your stone column is. If your computer is slower or faster your could change the column size and hight. Once it has finished loading do the same thing again until your mountain is filled. You can cover it in dirt the same way you would an arch. Then use [t//Br smooth] and bind it to a tool. Smooth out your mountain. For snow use [t//snow x]. X represents the radius. Making a volcano is quite similar, you will want to make a bigger base proportionally though. Fill it in, but leave some space at the top. Take some time to smooth out the volcano. You can make a hole like you would normally, or use [t//Br sphere air 1]. Bind that to a tool. The hole at the top should be about 10-20 blocks across and 5 deep. Around the edge of the hole make a stone rim. Stand on the desired level you want the lava and use [t//fill lava 20]. Now you can add the lava flows. Don't add too much! Placement: Volcanos go best on large islands and in mountain ranges. Mountains can go just about anywhere.
Islands: Islands are one of the hardest things to terraform. If you don't know what you are doing just use WorldPainter to make a simple island and then terraform the rest. Also you can try to find an existing island and terraform it. It is very hard to terraform in water.
Caves: With these mods you can make excellent caves of varying length. Use [t//Br sphere air x]. X is the size you want for your cave. When making your cave make sure it branches out and has rooms of varying size. Also a large room with water on the bottom looks nice. Be careful, it is easy to get lost!
Scenery: No map is complete without some nice scenery to complete the effect. Make sure your terraforming fits in with the surroundings. You can use [t//snow x] for snow. X is for the radius of the snow. Placing vines and lily pads around your lake can add a nice tropical touch. Also be sure to add lots of vegetation. Don't place too many vines and take the time to let them grow. Its nice to place trees everywhere and waterfalls. Once again don't place to many waterfalls.
Examples: -The Basics- -Volcano Tutorial- -Arches Tutorial- -Island Tutorial- -Floating Island Tutorial-

Ideas: When making maps you will often

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