Look at the wiki for the Sub8 github repository. It will walk you through what you need to do/know.

Sub8 has many dependencies, but we have a convenient install script. If you don't like where the dependencies are automatically installed (repos/sub8_dependencies), or where the catkin workspace is made (~/repos/catkin_ws), then you can change the variables set in the script.

I suggest you do the following (Don't run as sudo!):

wget -O install_sub8.sh https://github.com/uf-mil/Sub8/blob/master/install.sh
chmod +x ./install_sub8.sh
./install_sub8.sh

You can just do (Don't run as sudo!)

./install.sh

In the Sub8 root directory. It will check if you have dependencies installed and not install them if you already have them

The install script is intended to handle every single thing that needs to be installed to run the sub. If it does not work, something has gone wrong that we need to fix. If you fix an issue while installing, please fix the install script and submit a pull-request with your changes.

The install script can accept arguments. ./install ~/repos/deps ~/mil_ws will install the sub dependencies in ~/repos/deps and generate a catkin workspace with Sub8 in it at ~/mil_ws. It will make all of the directories it needs