It often depends on how loose your trucks are or even how wide your trucks are too, but given you have trucks that are not quite so wide, either option would work.
Pretty much been said, but the two options:
One side - usually heel side, so ollie on frontside and keep all your weight over the top of your heels and sit with both trucks to your wheels with minimum weight on your toes, usually a frontside exit as well.
Cross locked - usually back heel and front toe side on the rail, but some do it the other way round if you are more balanced over your front, but it is a little more balanced to stay upright and on top of the rail, easy enough to get on frontside or backside, but lean a bit more on the side you want to exit on.
A couple of other options just to get more comfortable are feeble or smith grinds, which you can usually sit on more easily than a straight fifty, especially feeble grinds, but sometimes people prefer fifties to anything else.
Momentum is often key as well. Going faster means it is easier to stay on the rail, but it is also down to balance and getting there comfortably too.
For a few things we used to do, just having a signpost / coping piece on the ground was the easiest way to get used to getting on a flat round rail, then very minimally put it up on something, so there is less chance of slipping off if it is more than a curb height.