You don't truly "install" Eaglercraft 1.8.8 like traditional software—you simply open the file. If you have the HTML file saved, you are done.
As of 2025, the original Eaglercraft 1.8.8 is no longer actively updated by its original creator (LAX1DUDE moved on to other projects). However, the community has forked it into several variants: eaglercraft 188 client install
More resources available at help.procreate.com