I’m not the best at doing backgrounds to be honest, but do take a look at this:
http://danfessler.com/blog/hd-index-painting-in-photoshop
This is an ideal technique for backgrounds, but it doesn’t work in aseprite, you’ll have to have photoshop for this (it doesn’t work in gimp or krita either, since they don’t support filter layers)
That is advanced stuff though, so don’t worry if you don’t get it. If you want, there is a PSD file in there, download and play around with it, see if you like it.
The traditional way of doing backgrounds it a bit trickier, and as I said, I’m not any good at it, so I’m probably not a better source than the tutorials you are watching.