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.
(https://rmrk.net/proxy.php?request=http%3A%2F%2Faycu22.webshots.com%2Fimage%2F15021%2F2004127069410836980_th.jpg&hash=9b159d46e1aeac3fbf7689eebda6805d58d4641d) (http://allyoucanupload.webshots.com/v/2004127069410836980)
Start off by making a jagged rock facing at the bottom, like this:
(https://rmrk.net/proxy.php?request=http%3A%2F%2Faycu22.webshots.com%2Fimage%2F15021%2F2004149791320911471_th.jpg&hash=4d6295e398eebb572ad5b5e9ee21728aaeab011c) (http://allyoucanupload.webshots.com/v/2004149791320911471)
You have to remember, there is no straight lines in nature.
After that make a stream and a rock facing that looks like this:
(https://rmrk.net/proxy.php?request=http%3A%2F%2Faycu35.webshots.com%2Fimage%2F12794%2F2004174644018123423_th.jpg&hash=210a2ccc2ea4d87b314149a7c3e63a055e18f090) (http://allyoucanupload.webshots.com/v/2004174644018123423)
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!
(https://rmrk.net/proxy.php?request=http%3A%2F%2Faycu11.webshots.com%2Fimage%2F12330%2F2004124110104816991_th.jpg&hash=f8ebb5c4e163769b64b652d70c6f3ce0dbbe563b) (http://allyoucanupload.webshots.com/v/2004124110104816991)
Now, go to layer 3 and finish the bridge rails, and if you like, overlap some trees to give it a cool effect.
(https://rmrk.net/proxy.php?request=http%3A%2F%2Faycu02.webshots.com%2Fimage%2F13921%2F2004196467594266090_th.jpg&hash=b073408d9cede468a55c10224b80bba1ca470f0a) (http://allyoucanupload.webshots.com/v/2004196467594266090)
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!!
(https://rmrk.net/proxy.php?request=http%3A%2F%2Faycu33.webshots.com%2Fimage%2F15752%2F2004179860589527515_th.jpg&hash=5bdb2624052d1eb2f046f512cc757fb3820eca08) (http://allyoucanupload.webshots.com/v/2004179860589527515)
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: Play BGS: '010-River01'. 80. 100.