By drawing the blade away from me diagonally along the top and then the bottom, to do both sides.
As a generally rule I usually just sharpen it before I cut tomatoes. This seems to be often enough.
EDIT:
I realise the first description is a bit shit.
Knife in right hand, steel in left pointing straight out as though you were going to stab someone with a thrusting motion.
start with the base of the blade almost against the base of the steel and draw the blade up and out in a diagonal motion, away from you and up the steel.
repeat with the other edge on the bottom of the steel.
By drawing the blade away from me diagonally along the top and then the bottom, to do both sides.
As a generally rule I usually just sharpen it before I cut tomatoes. This seems to be often enough.
EDIT:
I realise the first description is a bit shit.