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".

To just run Piskel @Jos solution works, but I found this annoying so I set it as an alias:

1. if not present create "bash_aliases" file in the home ("~") directory

2. add the alias, e.g.:

    alias piskel='pathtofolder/piskel'
   

3. open any terminal and type "piskel" and it should open.

4. To make it work in currently open terminals, type:

    source .bashrc
   

I still ran into the problem @Jos mentioned about adding it to the launcher, and when I pass it a .piskel file I get a error, but I hope this helps!

Sources: How can I create an alias so that when I enter "kt" it executes "killall gnome-terminal"?