Run this command (you need curl installed):
(Please make sure to run this command in the terminal inside the folder you want the game to be installed on)
bash <(curl -s https://raw.githubusercontent.com/nhaar/Waddle-Forever/master/linux.sh)
The game uses an AppImage, so you need to install FUSE in order to run it. In some Linux distributions, this already comes packaged, but if you don't have it, here's the tutorial for installing it.