What you need:
A Basic understanding of mapping in RMXP
RMXP(duh.)

Obviously, make a new map with the mountain tileset, You will see this.
http://aycu22.webshots.com/image/15021/2004127069410836980_th.jpg[/img]

Start off by making a jagged rock facing at the bottom, like this:

http://aycu22.webshots.com/image/15021/2004149791320911471_th.jpg[/img]

You have to remember, there is no straight lines in nature.
After that make a stream and a rock facing that looks like this:

http://aycu35.webshots.com/image/12794/2004174644018123423_th.jpg[/img]

I would have given you a screenshot of the process, but I messed up
Anyways, go to layer 2 and start on some of the plants and trees and add the bridge, but don't go overboard on the trees, we need some space for the hard part!

http://aycu11.webshots.com/image/12330/2004124110104816991_th.jpg[/img]

Now, go to layer 3 and finish the bridge rails, and if you like, overlap some trees to give it a cool effect.

http://aycu02.webshots.com/image/13921/2004196467594266090_th.jpg[/img]

Now for the hard part. Make two or more rock facings in the blanks you left in the trees. Remember, no straight lines in nature!!

http://aycu33.webshots.com/image/15752/2004179860589527515_th.jpg[/img]

Now that you are done wth that, finish up the mountainy feel by adding some more foliage such as flowers, more trees, and possibly a spring and then you have your mountain!
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Tips
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You might improve your bridge by adding some river Sound effects, it works like this:
Hope this helps you, feedback is much appriciated!