Hi again,
If you look at the diagram in my previous post, the force in the direction of the thumb is the force that a positive charge would experience. If you look at the particle traces at the bottom you'll see that a negative charge experiences the same force but it is in the opposite direction.
So changing only the direction of the force, if we orient the left hand the same way as the diagram shows the right hand then the only difference will be that the thumb points up instead of down, and that would mean the thumb would point in the direction of the force for a negative charge.
So use the left hand rule (if you care to) for negative charges and the right hand rule for positive charges. It's probably simpler just to use the right hand rule and just reverse the direction of the force for negative charges.
It's also possible to draw the three axises x, y, and z, in some standard position you are comfortable with and assign the velocity to one, the B field to another, and the force to the other. If we draw the three axises with x to the right, y up, and z out of the page, then x is the velocity, y is the force for a negative charge, and z is the B field. This is a good idea for people who have had double amputations too.