Inside a fish shell run:
source ~/fuchsia/scripts/fx-env.fish
fx-env.fish can be sourced within the shell as needed or added to your fish config. You can add it to your config with:
mkdir -p ~/.config/fish/conf.d/ echo 'source ~/fuchsia/scripts/fx-env.fish' >> ~/.config/fish/conf.d/fuchsia.fish
Replace ~/fuchsia above with the location of your fuchsia checkout.