I'm not amazing or anything but I can offer what few tips I have that have helped me so far

The best way to consistently rely on your lasso for building your magic meter is to work it into a combo, otherwise unless you can read your opponent you might struggle to properly land your hits (the hitbox is a bit deceptively small compared to the animation itself)

A good bread and butter to learn with Arizona is chaining forward light (A), forward heavy (C), and then low lasso to guarantee a hit if you're having trouble building magic through raw lasso grabs in neutral. I recommend checking out her combo trials in the training options to learn more, they're very helpful

Otherwise your best bet is to try and read your opponent as best as you can and practice your spacing. It can be tough at first but the more you work on it the more you'll start to understand the proper distance to keep between you and your opponent if you're fishing for lasso grabs. Be mindful of enemies that make a habit of jumping, and be mindful of your spacing

It's tricky for sure but you can get the hang of it with time. Also, note that the AI at higher difficulties will almost always perfectly avoid your lasso grabs so practicing against them can be frustrating (as I quickly learned), so it's better to lower their difficulty a tad if you're looking to practice spacing against them