Something fun to practice that will help with saving your second jump and controlling your float is to wd off of a ledge and immediately be in a float.
Begin by wding off of the ledges on pkmon stadium or ys (dreamland works, too of course, but they're too tall for my taste)
then while holding x, press down. To see if you did it correctly, try to double jump out of your float.
When you get good at this try fair immediately after this and dropping at the same time.
As a final test, try doing this off of the edge and immediately edge guarding a cpu or something. It's really dangerous, but can be rewarding.
Also practice basics:
chain grabbing fox, falco, and falcon on fd. Discover the correct percents to begin the cg.
quick fc nair's and bairs out of shield
double jump canceling out of shield ( don't do an attack, just double jump)
That's all i can think of right now. Also watch videos of pros playing against characters that give you difficulty. A vid is worth 1000 posts.