First of all you need to download single player commands, or get the world edit plugin if you are using a server.
First of all equip a tool of your choice and type the following. /brush sphere 1 5
Once you have done that, right click to place spheres. Do this until you have done the outline of your mountain.
Next build a tower up to where you want the summit to be.
From there go off the tower building down, or from the base building up.
Then start making spheres coming off that.
Made a stuff up??...
Just type //undo
Keep voxelin' :D
After a few minutes of right clicking, you will be finished the base. But that's not enough! It needs to be smoother.
Type the following to equip the smoothing tool. /brush Smooth 5
Start right clicking where you want it to be smoother.
If some bits need a touch up, equip a smaller sphere brush, like so, then smooth it again. The last number in the command is the brush size, the second last number is the block.
Once you are finished with the shape, select a corner by left clicking with the wooden axe (get it with //wand).
Then go to the opposite corner and make sure the whole mountain is between you and the first selection. Then type //hsphere 1 1
Select your second corner with a right click on the axe.
I hope this entertains you for a while! Thanks for watching :D