Using a crescent wrench, unscrew the top of the master...disconnecting any vent line connection first of course, then fill till its up to the top so that when you put the top back on, none spills out, just below the lip basically, and you'll be fine.
Due to the nature of brake fluid, except DOT5, absorping water/humidity, the fluid in there may be pretty gunky if it hasnt been flushed in the last year or 2...might be a good time to do that...manuals recommend every 2 years, completely flush all lines...if you reach in the bottom of the master with your finger and find sludge/sandy stuff, you know it needs it bad.
brute4c
brute4c