I've done editing for audio files, and it's pretty easy, but I've not done it for visuals. But I believe the process for removing unwanted clips or inserting should be similar. The process will depend on the software.
Regarding downloading youtube videos, I've never downloaded it coz' the quality's not that great. But there're a couple of threads in sajha teaching us downloading youtube videos. If you've missed, please follow the links:
I know more abt copy n' paste than other skills
http://www.sajha.com/sajha/html/OpenThread.cfm?forum=2&ThreadID=48820 Therewas one more thread, which I couldn't find now. There're other skilled sajha users who have better answers than mine.
good luck.