You are definitely getting the hang of it! Nothing is worse than putting one up against a black background like that. It can be hard to see light colored stuff when erasing on a png background. So many times have I made something and then pasted it on a background only to realize I missed a bunch of crap lol. It's all trial and error but that is really good for one of your first attempts. It does get easier the more you do it, you should see some of my early stuff it pains me to look at it ;)