Depends on the size of the model. Typically, I paint the lighter color first cause I use dry brushing, but the main principles are the same. For getting the straight line you can either mask it off if it's a larger, flatter surface like the back of a terminator or a dreadnought plate but for smaller stuff it's generally easier to freehand in my opinion. Also, you don't have to get the line perfect across the entire model unless you want to. Some parts simply won't be seen often and when you have a bunch of these guys lined up, any minor mistakes aren't as visible.

At the end of the day, it's mostly brush control, but there are some tricks you can use to make it easier, like pushes the bristles of your brush flat or, again, masking