Particles are immensely confusing, especially when just starting to learn how they work.  Their usefulness can not be overlooked, though, so wanted to at least share this bit on how to use them to build complicated scenes with little work.

I learned to do particles through getting the basics from Zachariah Scott (links below) and then playing with existing particles to see what happens.

OTHER USEFUL PARTICLE TUTORIALS:
How to add shadows to particles:
http://www.youtube.com/watch?v=fdQAzVM0BkY
Adding "wind effects" to a particle:
http://www.youtube.com/watch?v=xyHUuhCQO70

OTHER TUTORIALS FOR SFM!
http://www.youtube.com/playlist?list=PLmx5NIR6PO5t2vfHIX6GNkpXiFODM_6KJ

DOWNLOAD LINK
This is a basic version of the particle system, using TF2 trees (due to it not requiring outside resources). It will spam 10 trees in a small area. Modify to your liking:
https://www.dropbox.com/s/8kf0znm9navzkik/tree_spam.zip

Need more tree and grass models?
http://sfmbox.com/foliage-model-pack/

Map used:
http://steamcommunity.com/sharedfiles/filedetails/?id=207506735

My DeviantArt!
http://argodaemon.deviantart.com/

My Twitter!
https://twitter.com/argodaemon

Equestria Daily:
http://www.equestriadaily.com/2013/12/sfm-tutorials-magic-particle-effects.html

Background Music:
"Cipher" Kevin MacLeod (incompetech.com)
Licensed under Creative Commons: By Attribution 3.0
http://creativecommons.org/licenses/by/3.0/