If you aren't comfortable working with a command line, making and testing changes in Islandora 8 can be tricky. The best way to test out your changes is to use an Islandora Virtual machine built from the claw-playbook, but once you get your VM up and running, getting access to the file system via a GUI is a challenge that requires a brief stint into the command line. If you have a GUI client you like to use (Cyberduck, WinSCP, etc), you can't just point it to your localhost and browse the file syste of your VM's Drupal.
If you are using VirtualBox, you can use the terminal wndow for your VM to login. Username vagrant, password vagrant. Once logged in, we need to chage some things to allow access via the GUI client of our choice. Type sudo vi /etc/ssh/sshd_config and hit Enter. Using arrows keys to move your cursor around, scroll down until you see #PasswordAuthentication no or PasswordAuthentication no and change the no to yes, removing the # if it's there. Hit Escape to exit Insert mode, then type :x and hit enter, which should save your changes. Next, type sudo systemctl restart sshd and hit Enter.
You can now log into you VM using the GUI of your choice.
Now that the GUI can connect and make changes, navigate to the folder in Drupal that you want to update. In my exaple, this is var/www/html/drupal/web/modules/contrib/islandora_defaults/config/install. If the file you want to update exists, edit it. If it does not exist, create it.
You will need to visit the terminal window one more time to have your change take effect in your VM. enter the command cd var/www/html/drupal to navigate to the drupal directory, and then type drush fim -y islandora_core_feature and hit Enter. Go to your VM and try to add new content; your form changes should be there!