How is it done? Ive been experimenting with terragen recently and all i can do is normal mountains and play around with the clouds and water, havent been able to do anything really advanced
You should start with reading some tutorials first.
For starters here are some links to good tutorials:
The Official Terragen Frequently Asked Questions: http://www.planetside.co.uk/terragen/guide/tg_faq.html
Clouds: http://private.addcom.de/golem/tg/wolken/wolken.html (German)
Water: http://private.addcom.de/golem/tg/wasser/wasser.html (German)
Surface: http://oshyan.ashundar.co.uk/articles.html (English)
Official Surface Guide: http://www.planetside.co.uk/terragen/smgdl.shtml (English)
Carol Brooksbank's tutorial: http://www.caroluk.org.uk/terratut/ (English) Very extensiv!
JRSs Terragen™ tutorials: Zipped PDF file download (1,15 MB)
me109's Reference Guide for water setings: Water research
I'd advice Carol Brooksbank's tutorial to start with.
If you want snow, use a white surface layer on top of everything else. Make it kinda bumpy and set a slope limit.