This is the most fun way to terraform for me by far! TNT, C4, Grenades, and Boom arrows are like the carve tool of Total Miner!
There are a few exercises you can do to familiarize yourself with using explosives for terraforming.
-Start a new creative map, toggle combat off & dig a hole 15 blocks down. Plant a TNT, light it & observe the blast radius.
-Now move over 15 blocks, and repeat the experiment with C4.
-Move over 15 again, dig down 10 blocks, aim a Grenade launcher at the bottom block & take notice of the damage caused.
-Do the same using boom arrows.
The reason I asked you to do this is so you can observe the blast radius of these tools. This will come in handy when doing the following exercises;
First : Make a river-Using explosives, mark the course of the center of your river, keeping in mind the blast radius of both TNT and C4. Make sure to stack the explosives accordingly.
Usually, TNT on the third block above the center line will explode a layer off of the ground. Using that info, use the explosives to carve your river.
After it's all done replace the exposed layers with whatever material your using for the river and voila! ..Add any details you learned when hand terraforming a river and you got a semi-quick way to a decent river!
Second: Make giant blocks of varying heights and widths Connect them or leave a small gap. Build it up as big as you want, or not! Just make sure that the cubes that are stacked are a couple of blocks smaller than the one below it.
-(pro-tip: don't make it too big or your gonna regret it! >,<)When you have the basic shape down, get a Launcher and bow on your hotbar. You're gonna explode the corners! Yay!
Go around the chunks of rock with the launcher first. The corners will have floating blocks as a result of the explosion, disregard that, you'll fix it later!
Once all the corners have been "smoothed", start "smoothing" the hard edges of the chunks.
Now using boom arrows, smooth the edges and connect any slope-like ridges.
When you're done running around exploding rocks like a madman
(+cred for wearing the Madman Skin! >,<), get a hammer and remove any floating blocks, & fill in any single holes.
Smooth the areas till you're as pleased as a peach!
After all the explosions and smoothing out are done, use whatever tools to add details to your new terrain feature - I like making jaggy hills near the base of these kinds of mountain; pepper the base with scoria, dirt and whatever block you used to make this feature.
Flood, Fill, Replace: They're not useless!
There's a quick guide on the forums explaining how to use the markers that are needed for these creative functions.
Check it out
here if you don't know what they are.
The rest of this guide will assume you have familiarised yourself with them.
Flood: This here feature is stupid useful if your like me and tend to spruce up your ocean floors before adding the water in 'em.
In relation to terraforming though, this tool is pretty self explanatory.. You got a few odd looking holes you want filled, pop a marker at the height you need it filled in, select the block and BAM! Instant(ish) flood fill!
The Flood tool can use almost any block so don't forget about using this feature when making rivers or oceans and anything else that needs a bit of filling.
Fill: Hoo-boy! This little gem has been around for awhile, does it even need explaining at this point?! Either way this tool comes in handy for filling in those queer holes as well as the flood feature.
Unlike the flood feature, fill only shoves blocks in the pre-defined area set up by markers. So, ya got it yet?
Replace: Oh Baby! This is my favorite tool in Total Miner, right up there with editing NPC text and the Books! Super useful and uber fun when used in conjunction with existing System Terrain tools. Neglect this tool and-and well your a tool!

In all seriousness though, I found that using the Replace Feature with my terrain kit allows for some pretty darned awesome biome types! Replace the grass and grassy stones on any of the Grassy Transitionals, and BOOM, Instant Desert/Wasteland biome tool! -Or better yet, swap the Snow out with Sand, the Diotite with Sandstone on the Snowy Rock Tools and Presto, Insta(ish) Mesa tool!
I love this feature and I swear to god I'll box your ears if I find out you read this whole guide and still complain about lack of biome tools in my packs! (Harrumph to all you haters!)