ATG under the chin and on the neck is usually the hardest part of straight shaving to master, so don't despair if it takes some time! Some people dispense with it altogether - if your skin is very sensitive then the discomfort might not be worth it. Might be worth practising on your cheek for a bit rather than getting your neck irritated.
A lot of the problem can be solved by finding which way the grain actually lies - in these areas the grain is more prone to lie randomly, some small areas pointing one way and areas right by them pointing another way. You can feel this with your fingertips - when you encounter most resistance you are against the grain. Remembering what grows in which direction can be a bit of a memory feat, though. It isn't automatically down-up - it could be at any of a number of angles, unless you have very well-behaved whiskers!
You can also help yourself by knowing what happens when you stretch the skin - if you stretch the skin with the grain the hairs will lie down, stretch it against the grain and they will stand up, so you could find yourself stretching the skin immediately in front of the blade to get it to stand up a bit and easier to cut.
Next is the angle. A steeper angle is called for with the grain, say between 15 and 30 degrees. This is because you want the blade to bite straight away - if the angle is too shallow it will skip. If too much it will be very uncomfortable and/or you will dig a nice line in your face. Reduce the angle to around 15 degrees XTG and reduce it again to around 5 degrees for ATG. Start by laying the blade flat and lifting it slightly until it begins to cut. If you have to push - DON'T! - somethings not quite right. You may need more stropping, more experience or re-honing. Generally though, if the razor cuts well WTg and XTG it is a matter of technique.
Other factors that help some people are prep (use a hot towel to soften the whiskers); the way you hold the razor; the type of blade (full-hollow/half-hollow/quarter hollow/wedge); and whether the razor has a "smile" (ie a curved edge, widest at the belly, thinner at either end) or not - smiling razors are often said to me more comfortable to use; the type of stroke (try short scything or buffing motions for instance); stretching the skin past obstacles - eg pulling the skin on your chin lower so it is flatter and pulling the skin off the adams apple to one side or another to get a flatter surface.
Good luck!