How do I set permissions on a directory so that all files created there will be read/write/execute for everyone?
Right click the directory or Command + I after clicking on it, there will pop up a window like this. 

Clink on the padlock in lower right corner and type your password. Now you can change permissions for this directory. If you want everyone to be able to read and write, click on the privilege provided to Everyone and there will appear a dropdown menu from which you can choose what permission to provide.


I hope that helped.