command + S
Press and hold the Command modifier key, aka Apple key - located on either side of the spacebar - and then press the letter S. As soon as this keyboard shortcut is recognized on the first try, a pop-up window will appear to let you choose the title of your project file and the drive location for where it will save. You may release the keys when this happens so that you may title your project file.
After choosing the appropriate location to place your project file, click the Save button to continue working. The pop-up window will disappear after the save command, and then you will return to the window arrangement last seen while working.