Scale
Most likely I'll consider a square to be 1 yard (matching the gurps hex scale), but I will sometimes use 2 yards per square (the d&d scale is 1.5 yard per hex, which I think would be too troublesome.) When I do use 2 yards per square, it, in effect, costs one movement to enter a square, and another to leave it.
Reach/Movement/Range
Use the old d&d 3.5 rules. I.e. count squares, every second diagonal counts as 2.
Orientation/Facing/Arc of Vision (B388-389)
Basically, consider the square to be an eight-sided hexagon, an octagon.
To put it in words, three squares are to the Front, two squares are to the Left, one square is to the Back, two squares are to the Right. The three squares next to the Back square towards the 'back' and a straight line from that is not possible to attack unless you have 360-degree vision. See text and diagrams below for an attempted explanation. :)
In both diagrams below, '.' means the character can attack into it (given enough reach, and without using a Wild Swing), '!' means a character with Peripheral Vision can attack into it, '?' means characters with 360-degree Vision can attack into it.
I.e., if the character (the @ symbol) faces forward (towards the X):
....X....
...FFF...
...L@R...
!!!LBR!!!
!!!???!!!
!!?????!!
!???????!
And if he faces to the 'northeast' (towards the X):
!!!.......
!!!!....X.
!!!!!LFF..
!!!!!L@F..
?????BRR..
??????!!..
??????!!!.
??????!!!!
Mounts/Multi-hex monsters
Most mounts are 2-square creatures, the rider is normally in the rear square. For three-square and larger mounts, the rider must choose which square to be in.
Other rules
All other rules are likely to remain unchanged, if not they'll be added here as we discover them.