For someone who has studied standard Japanese, how hard is it to learn Kansai dialect? Is there a lot to learn, or not much? What are the basic things to learn in order to speak in this dialect? All I know so far is that the sentence ending particles are different.
Many people who spent years studying a language something not being able to learn anything. Your question's logic is not well formed.
Re-learn bits of grammar, vocabulary, and change completely your intonation.
なんでやねん！ もうかりまっか？ おおきに！ And fluency in standard Japanese too, because unless you're Daniel Kahl or someone like him, you'd better be able speak in standard Japanese when you're not with your friends anymore…
There's a lot more… If you want to have an idea of what it's like, read any volume of ナニワ金融道
'Is there a lot to learn?' is a pretty subjective question. You can learn as much or as little as you like, and different Kansai folks speak different levels of the dialect. My wife is from Kansai, so I picked it up from her and her family. I think that that would be the best way to go - find someone from Kansai and learn from them. Books are available, but I think it would be difficult to learn Kansaiben solely from a book.
Just to get you started, here's a little morsel:
あかん is used in place of だめ. My kids know this one well.
Apart from the fact that particles and verb/adjective endings differ, and the fact that vocabulary and usage also often differ -- both of which are not that hard to acquire and lots of information can be found online -- the single most difficult part will be to get the pitch accent right. It's one thing to have an accent in a Standard language, something people are used to, but you'll have to do fairly well for people to not find it annoying to hear you speak in another dialect. The tolerance level is much lower, unless you are with very close friends.
Very few foreigners bother to try to get pitch accent right in Standard Japanese to begin with, and info on Kansai-ben accent is much harder to find; I don't know of any definitive source on Kansai pitch like the Shinmeikai or NHK accent dictionaries for Standard. I don't think it would be reasonable to think you could learn to speak Kansai-ben correctly until you've got a good grasp on Standard pitch.
However, once you've got the basic grammar and common words down, understanding Kansai-ben is not that hard, provided your Standard Japanese level is already fairly advanced.