Well, if you're trying to make something more natural for you, to integrate it into your game so that you don't need to think about doing it, you can just do it instinctively, you simply need to start over-using it.
Maybe this isn't always a great rule of thumb, but in my own experience from Melee, when you're learning something new, you need to get used to it. You need to make it come naturally to you, that much is obvious. What worked best for me was simply using the HELL out of whatever I was trying to learn for a period of time. Just shield a LOT. Grab a LOT. Do it constantly. Do it a lot. Lose, win, it doesn't matter. Just do it a ton. Learn when it works and when it doesn't. Understand how to avoid getting punished for predictability, too. If you shield a lot, people are going to start trying to grab you, obviously. So practice avoiding that. Practice spotdodges out of a shield followed by a counterattack on your own part.
After you've done this for a while, and you've really gotten to know the in's and out's of what your trying to learn, start playing normally again. Don't think so much about shielding or grabbing, just play however feels natural. You'll start to see opportunities presenting themselves. You'll think "oh, I should've grabbed there" or "darn, why didn't I shield?" and pretty soon, you'll start taking advantage of those situations like you should.
Obviously, the most important part about learning anything is knowing how to use it in moderation. You want to be varied, you want to utilize every tool available to you. This kind of understanding only comes through time, and if you consciously overuse what you're trying to learn, you'll soon learn when it works and when you get punished.
You can only learn through experience, so the only good advice anyone can give is just to get out there and start trying. Think about it, intend to shield, intend to grab. Do it willfully, and think about the results. Do your best to understand why you succeed and why you fail. Through experience, you'll gain the ability to do it without so much intent--to do it instinctively. And then you're goal's reached.
Hope this helped.