Pick a time that you'll check the time on your grandfather clock. You'll have to do this every day for six days. Record the time on a piece of paper and check it against your own watch. Set the time on the grandfather clock to exactly the same time as is on your watch.
Check the time shown on the grandfather clock every day at the same time according to your watch. Write down the time on the clock and on your watch.
Turn the adjustment nut on the bottom of the pendulum one full counterclockwise turn for each half minute that the grandfather clock is fast.
Turn the adjustment nut on the bottom of the pendulum one full clockwise turn for each half minute that the grandfather clock is slow.
Check the time on your watch. Reset the time on the grandfather clock to match the watch. Continue to check the grandfather clock for six days to ensure that it's keeping the correct time. If not, repeat the steps.