I do my shields (or start them) with visualization—kind of. Basically, what I do is project or extending my energy around my body in the form of something I think of as protective. That can be a wall, a tent, an umbrella, a giant hamster ball—for myself it’s a ring of fire. Just focus on pushing your energy out, but not letting it go all the way.
If you’re familiar with spirit walking and astral travel, it’s a little like that. You’re pushing or expanding your energy around your body. The difference is, you’re not letting it go.
It’s easiest if you work on this a little bit at a time. Take a week to work on expanding your energy. Take another to settle on a protective form for your shield to take. Take a third to perfect that form. After that, you can tie your shield to a phrase or word if you want.
Another good way to do this would be with a guardian, which you can tie to an object you’ll keep on yourself and release when needed.