This raises more questions (sorry, I'm a noob at this).
1. What if I breed a Pokemon with a different M/F ratio?
2. How do I ensure the ability I want? (Ex. Magikarp has 1 natural ability but Ralts has 2 OR I breed using non-HA Magikarp to begin with but want a HA Eevee)
1. Flarg already answered this but basically you breed according to the F/M ratio, if using a another 50% like magikarp you'll get the same gender on said species. If on the other hand you're looking to make a 12.5% female you should be using a togetic to check instead (lowest step count of all 12.5% females iirc).
2. If you want to ensure what ability you'll get and don't mind riding the bike a bit more I suggest you use a Croagunk (50% F/M ratio with 2 abilities, can be caught on daycare route) or a Togetic (12.5% female ratio with 2 abilities as well). Jus take note if the ability changes or not, I haven't tried with HA yet but it should work under the same principle.