Tim, you first find the military map with the UTM grid "YA" on it to get into the general area.
YA646508
Break the above down as follows:
YA
64
6
50
8
Always read UTM grid locations "RIGHT THEN UP"
Find the block between lines 64 & 65, then go to where it intersects with lines 50 & 51. That's your 1000-meter grid square, 6450. A "klick" or a kilometer square.
Again, "RIGHT THEN UP," divide that square into tenths, mentally. Go 6/10s across the grid to the RIGHT, then 8/10ths UP. You have a 100-meter square area. YA646508. The marked grid would be YA6450.
GPS is now bringing it down finer than that, obviously, but the principal is the same.
This won't come out to scale, but here's the idea:
. . . . . x . . . . .
. . . . . . . . . . .
. . . . . o o . . . .
. . . . . o o . . . .
. . . . . . . . . . .
x . . . . x . . . . x
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . x . . . . .
The area inside the o's is your 6/10ths RIGHT and 8/10ths UP.
Bob K