How do I install/run PISKEL after it was downloaded?

Running it is straightforward.

1. Double click the downloaded zip file and unzip the package to your home directory. This will create a directory Piskel-0.11.0-64bits.
2. Open a terminal (CTRL + ALT + T).
3. Entercd Piskel-0.11.0-64bits.
4. Enter ./piskel.

The Piskel window appears. Unfortunately, this program does not follow application standards so there is no easy way to add this to your launcher.

You need to tell the operating system where to look for the piskel command. In this case, that is the current directory, indicated by .. So the command ./piskel means "look in the current directory and execute the program piskel located there".