So, if you are taking a Grid bearing from the map and wish
to convert it to a Magnetic bearing (as in Lesson Two on the
website*), you add the magnetic variation.
If you are taking a Magnetic bearing from the landscape and
wish to convert to a grid bearing (as in Lesson One on the
website*), you subtract the magnetic variation.
This only works if you are in a part of the world where
Magnetic North is west of Grid North as is the case in
Europe. If Magnetic North for your location is to the East
then you have to do the reverse of this and possibly come up
with your own rhyme!
*Ignore magnetic variation in the lessons on the website